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The Boeing B-17 Flying Fortress was a four-engine heavy bomber aircraft developed in the 1930s for the United States Army Air Corps (USAAC). Competing against Douglas and Martin for a contract to build 200 bombers, the Boeing entry outperformed both competitors and more than met the Air Corps' expectations. Although Boeing lost the contract because the prototype crashed, the Air Corps was so impressed with Boeing's design that they ordered 13 more B-17s for further evaluation. From its introduction in 1938, the B-17 Flying Fortress evolved through numerous design advances.

The B-17 was primarily employed by the United States Army Air Forces (USAAF) in the daylight precision strategic bombing campaign of World War II against German industrial and military targets. The United States Eighth Air Force based at Thorpe Abbotts airfield in England and the Fifteenth Air Force based in Italy complemented the RAF Bomber Command's nighttime area bombing in Operation Pointblank to help secure air superiority over the cities, factories and battlefields of Western Europe in preparation for Operation Overlord. The B-17 also participated to a lesser extent in the War in the Pacific where it conducted raids against Japanese shipping and airfields.

From its pre-war inception, the USAAC (later USAAF) touted the aircraft as a strategic weapon; it was a potent, high-flying, long-range bomber that was able to defend itself, and to return home despite extensive battle damage. It quickly took on mythic proportions, and widely circulated stories and photos of B-17s surviving battle damage increased its iconic status. With a service ceiling greater than any of its Allied contemporaries, the B-17 established itself as an effective weapons system, dropping more bombs than any other U.S. aircraft in World War II. Of the of bombs dropped on Germany by U.S. aircraft, 640,000 tons were dropped from B-17s.

B-17 Documents

  • B17 Pilot Flight Manual- This is a 25M pdf. You many need to right click, save it to your computer and open it from there.
  • B-17F Erection and Maintenance Manual- Handling and ground instructions. Very detailed with diagrams and photos. This is a 51M pdf. You many need to right click, save it to your computer and open it from there.

Design and development

On 8 August 1934, the U.S. Army Air Corps (USAAC) tendered a proposal for a multi-engined bomber to replace the Martin B-10. The Air Corps were looking for a bomber capable of reinforcing the air forces in Hawaii, Panama, and Alaska. Requirements were that it would carry a "useful bombload" at an altitude of for 10 hours with a top speed of at least . They also desired, but did not require, a range of and a speed of. The competition for the Air Corps contract would be decided by a "fly-off" between Boeing's design, the Douglas DB-1 and the Martin Model 146 at Wright Field in Dayton, Ohio.

The prototype B-17, designated Model 299, was designed by a team of engineers led by E. Gifford Emery and Edward Curtis Wells and built at Boeing 's own expense. It combined features of the experimental Boeing XB-15 bomber with the Boeing 247 transport airplane. The B-17's armament consisted of up to of bombs on two racks in the bomb bay behind the cockpit, and five machine gun s, and it was powered by four Pratt & Whitney R-1690 "Hornet" radial engines each producing at . The first flight of the Model 299 was on 1935, with Boeing chief test-pilot Leslie Tower at the controls. Richard Williams, a reporter for the ''Seattle Times'' coined the name "Flying Fortress" when the Model 299 was rolled out, bristling with multiple machine gun installations. Boeing was quick to see the value of the name and had it trademarked for use. On, the prototype flew from Seattle to Wright Field in nine hours and three minutes at an average cruising speed of, much faster than the competition.

At the fly-off, the four-engine Boeing's performance was superior to those of the twin-engine DB-1 and Model 146. Then-Major General Frank Maxwell Andrews of the GHQ Air Force believed that the long-range capabilities of four-engine large aircraft were more efficient than shorter-ranged twin-engined airplanes, and that the B-17 was better suited to their doctrine. His opinions were shared by the Air Corps procurement officers, and even before the competition had finished they suggested buying 65 B-17s.

Development continued on the Boeing Model 299, and on 1935, Army Air Corps test-pilot Major Ployer Peter Hill and Boeing employee Les Tower took the Model 299 on a second evaluation flight; however, the crew forgot to disengage the airplane's "gust lock," a device that held the bomber's movable control surfaces in place while the aircraft was parked on the ground. Having taken off, the aircraft entered a steep climb, stalled, nosed over and crashed, killing Hill and Tower (other observers survived with injuries). The crashed Model 299 could not finish the evaluation, and while the Air Corps was still enthusiastic about the aircraft's potential, Army officials were daunted by the much greater expense per aircraft (Douglas quoted a unit price of $58,200 based on a production order of 220 aircraft, compared with a price of $99,620 from Boeing), and as the competition could not be completed Boeing was legally disqualified from the consideration for the contract. Army Chief of Staff Malin Craig cancelled the order for 65 YB-17s, and ordered 133 of the twin-engine Douglas B-18 Bolo instead.

Regardless, the USAAC had been impressed by the prototype's performance, and on 1936, through a legal loophole, the Air Corps ordered 13 YB-17s (designated Y1B-17 after November 1936 to denote its special F-1 funding) for service testing. The YB-17 incorporated a number of significant changes from the Model 299, including more powerful Wright R-1820 -39 Cyclone engines replacing the original Pratt & Whitneys. Although the prototype was company-owned and never received a military serial (the B-17 designation itself did not appear officially until January 1936, nearly three months after the prototype crashed), the term "XB-17" was retroactively applied to the airframe and has entered the lexicon to describe the first Flying Fortress.

Between 1 March and 4 August 1937, 12 of the 13 Y1B-17s were delivered to the 2nd Bombardment Group at Langley Field in Virginia for operational development and flight tests. One suggestion adopted was the use of a checklist to avoid accidents such as that which befell the Model 299. In one of their first missions, three B-17s, directed by lead navigator Lieutenant Curtis LeMay, were sent by General Andrews to "intercept" and photograph the Italian ocean liner ''Rex'' off the Atlantic coast. The mission was successful and widely publicized. The 13th Y1B-17 was delivered to the Material Division at Wright Field, Ohio, to be used for flight testing.

A 14th Y1B-17 (''37-369''), originally constructed for ground testing of the airframe's strength, was upgraded and fitted with exhaust-driven turbochargers. Scheduled to fly in 1937, it encountered problems with the turbochargers, and its first flight was delayed until 1938. The aircraft was delivered to the Army on 1939. Once service testing was complete, the Y1B-17s and Y1B-17A were redesignated B-17 and B-17A respectively to signify the change to operational status.

Opposition to the Air Corps' ambitions for the acquisition of more B-17s faded, and in late 1937, 10 more aircraft designated B-17B were ordered to equip two bombardment groups, one on each U.S. coast. Improved with larger flaps, rudder and Plexiglas nose, the B-17Bs were delivered in five small batches between July 1939 and March 1940. In July 1940, a significant order for 512 B-17s was issued; however, prior to the attack on Pearl Harbor, fewer than 200 B-17s were in service with the Army. A total of 155 B-17s of all variants were delivered between 1937 and 1941, but production quickly accelerated with the B-17 eventually setting the record for achieving the highest production rate for large aircraft. The aircraft went on to serve in every World War II combat zone, and by the time production ended in May 1945, 12,731 aircraft had been built by Boeing, Douglas, and Vega (a subsidiary of Lockheed).

Operational history

The B-17 began operations in World War II with the RAF in 1941, and the USAAF Eighth Air Force and Fifteenth Air Force units in 1942. It was primarily involved in the daylight precision strategic bombing campaign against German industrial targets. In the campaign against German aircraft forces in preparation for the invasion of France, B-17 (and B-24 Liberator ) raids were directed against German aircraft production while their presence drew the ''Luftwaffe'' fighters into battle with Allied fighters.

Early models proved to be unsuitable for combat use over Europe and it was the B-17E that was first successfully used by the USAAF. The defense expected from bombers operating in close formation alone did not prove effective and the bombers needed fighter escorts to operate successfully.

During World War II, the B-17 equipped 32 overseas combat groups, inventory peaking in August 1944 at 4,574 USAAF aircraft worldwide. B-17s dropped of bombs on European targets (compared to dropped by the Liberator and dropped by all other U.S. aircraft). The British heavy bombers, the Avro Lancaster and Handley Page Halifax, dropped 608,612 and 224,207 long tons respectively.

The RAF

The Royal Air Force (RAF) entered World War II with no heavy bomber of its own in service; the biggest available were long-range medium bombers such as the Vickers Wellington which could carry up to 4,500 lb of bombs. While the Short Stirling, and Halifax would become its primary bombers by 1941, in early 1940 the RAF entered into an agreement with the U.S. Army Air Corps to be provided with 20 B-17Cs, which were given the service name Fortress I. Their first operation, against Wilhelmshaven on 1941 was unsuccessful, and on, the target was Brest, France, but again the bombers missed completely.

By September, after the RAF had lost eight B-17Cs in combat or to accidents and many instances of aborts due to mechanical problems, Bomber Command abandoned daylight bombing raids because of the Fortress I's poor performance. The experience showed both the RAF and USAAF that the B-17C was not ready for combat, and that improved defenses, larger bomb loads and more accurate bombing methods were required. However the USAAF continued using the B-17 as a "day" bomber, despite pleas from the RAF that attempted daylight bombing would be ineffective.

As usage by Bomber Command had been curtailed, the RAF transferred its remaining Fortress I aircraft to Coastal Command for use as a long-range maritime patrol aircraft instead. These were later augmented in August 1942 by 19 Fortress Mk II and 45 Fortress Mk IIA. A Fortress from No. 206 Squadron RAF sank U-627 on 1942, the first of 11 U-boat kills credited to RAF Fortress bombers during the war.No. 223 Squadron, as part of 100 Group operated a small number of Fortresses in support of the bombing offensive for jamming German radar.

Initial USAAF operations over Europe

The Air Corps (renamed United States Army Air Forces or USAAF in 1941), using the B-17 and other bombers, bombed from high altitudes using the then-secret Norden bombsight, which was an optical electro-mechanical gyro-stabilized analog computer . The device was able to determine, from variables input by the bombardier, the point at which the aircraft's bombs should be released to hit the target. The bombardier essentially took over flight control of the aircraft during the bomb run, maintaining a level altitude during the final moments before release.

The USAAF began building up its air forces in Europe using B-17Es soon after entering the war. The first Eighth Air Force units arrived in High Wycombe, England, on 1942, to form the 97th Bomb Group. On 1942, 12 B-17Es of the 97th, with the lead aircraft piloted by Major Paul Tibbets and carrying Brigadier General Ira Eaker as an observer, were escorted by RAF Spitfires on the first USAAF raid over Europe, against railroad marshalling yards at Rouen -Sotteville in France, while a further six aircraft flew a diversionary raid along the French coast. The operation was a success, with only minor damage to two aircraft.

As the raids of the American bombing campaign grew in numbers and frequency, German interception efforts arose to respond to the bombers (such as during the attempted bombing of Kiel on 13 June 1943) to the level where unescorted bombing missions became discouraged.

Combined offensive

Since the airfield bombings were not appreciably reducing German fighter strength, additional B-17 groups were formed, and Eaker ordered major missions deeper into Germany against important industrial targets. The 8th Air Force then targeted the ball-bearing factories in Schweinfurt, hoping to cripple the war effort there. The first raid on 1943 did not result in critical damage to the factories, with the 230 attacking B-17s being intercepted by an estimated 300 ''Luftwaffe'' fighters. The Germans shot down 36 aircraft with the loss of 200 men, and coupled with a raid earlier in the day against Regensburg, a total of 60 B-17s were lost that day.

A second attempt on Schweinfurt on 14 October 1943 would later come to be known as "Black Thursday ". While the attack was successful at disrupting the entire works, severely curtailing work there for the remainder of the war, it was at an extreme cost. Of the 291 attacking Fortresses, 60 were shot down over Germany, five crashed on approach to Britain, and 12 more were scrapped due to damage – a total loss of 77 B-17s. One hundred and twenty-two bombers were damaged and needed repairs before their next flight. Out of 2,900 men in the crews, about 650 men did not return, although some survived as prisoners of war. Only 33 bombers landed without damage. These losses were a result of concentrated attacks by over 300 German fighters.

Such high losses of air crews could not be sustained, and the USAAF, recognizing the vulnerability of heavy bombers to interceptors when operating alone, suspended daylight bomber raids deep into Germany until the development of an escort fighter that could protect the bombers all the way from the United Kingdom to Germany and back. At the same time, the German night fighting ability noticeably improved to counter the nighttime strikes, challenging the conventional faith in the cover of darkness. The Eighth Air Force alone lost 176 bombers in October 1943, and was to suffer similar casualties on 1944 on missions to Oschersleben, Halberstadt and Brunswick . Lieutenant General James Doolittle, commander of the Eighth, had ordered the second Schweinfurt mission to be cancelled as the weather deteriorated, but the lead units had already entered hostile air space and continued with the mission. Most of the escorts turned back or missed the rendezvous, and as a result 60 B-17s were destroyed. A third raid on Schweinfurt on 1944 highlighted what came to be known as "Big Week ", during which the bombing missions were directed against German aircraft production. German fighters would have to respond, and the North American P-51 Mustang and Republic P-47 Thunderbolt fighters (equipped with improved drop tank s to extend their range) accompanying the American heavies all the way to and from the targets would engage them. The escort fighters reduced the loss rate to below seven percent, with only 247 B-17s lost in 3,500 sortie s while taking part in the Big Week raids.

By September 1944, 27 of the 40 bomb groups of the Eighth Air Force and six of the 21 groups of the Fifteenth Air Force used B-17s. Losses to flak continued to take a high toll of heavy bombers through 1944, but by 1945 (two days after the last heavy bombing mission in Europe), the rate of aircraft loss was so low that replacement aircraft were no longer arriving and the number of bombers per bomb group was reduced. The Combined Bomber Offensive was effectively complete.

Pacific Theater

Before the advent of long-range fighter escorts, B-17s had only their abbr=on}} M2 Browning machine gun s to rely on for defense during the bombing runs over Europe. As the war intensified, Boeing used feedback from aircrews to improve each new variant with increased armament and armor. The number of defensive guns increased from four machine guns and one nose machine gun in the B-17C, to thirteen machine guns in the B-17G. But because the bombers could not maneuver when attacked by fighters, and during their final bomb run they needed to be flown straight and level, individual aircraft struggled to fend off a direct attack.

A 1943 survey by the Air Corps found that over half the bombers shot down by the Germans had left the protection of the main formation. To address this problem, the United States developed the bomb-group formation, which evolved into the staggered combat box formation where all the B-17s could safely cover any others in their formation with their machine guns, making a formation of the bombers a dangerous target to engage by enemy fighters. ''Luftwaffe'' "''jagdflieger''" (fighter pilots) likened attacking a B-17 combat box formation to encountering a ''fliegendes stachelschwein'', or "flying porcupine". However, the use of this rigid formation meant that individual aircraft could not engage in evasive maneuvers: they had to always fly in a straight line, which made them vulnerable to the German flak. Additionally, German fighter aircraft later used the tactic of high-speed strafing passes rather than engaging with individual aircraft to inflict damage with minimum risk.

As a result, the B-17s' loss rate was up to 25% on some early missions (60 of 291 B-17s were lost in combat on the second Raid on Schweinfurt ), and it was not until the advent of long-range fighter escorts (particularly the North American P-51 Mustang ) resulting in the degradation of the ''Luftwaffe'' as an effective interceptor force between February and June 1944, that the B-17 became strategically potent.

The B-17 was noted for its ability to absorb battle damage, still reach its target and bring its crew home safely. Wally Hoffman, a B-17 pilot with the Eighth Air Force during World War II, said, "The plane can be cut and slashed almost to pieces by enemy fire and bring its crew home."Martin Caidin reported one instance in which a B-17 suffered a mid-air collision with a Focke-Wulf Fw 190, losing an engine and suffering serious damage to both the starboard horizontal stabilizer and the vertical stabilizer, and being knocked out of formation by the impact. The airplane was reported as shot down by observers, but it survived and brought its crew home without injury. Its toughness more than compensated for its shorter range and lighter bomb load when compared to the Consolidated B-24 Liberator or the British Avro Lancaster heavy bombers. Stories abound of B-17s returning to base with tails having been destroyed, with only a single engine functioning or even with large portions of wings having been damaged by flak . This durability, together with the large operational numbers in the Eighth Air Force and the fame achieved by the "Memphis Belle ", made the B-17 a significant bomber aircraft of the war. Other factors such as combat effectiveness and political issues also contributed to the B-17's success.

The B-17 design went through eight major changes over the course of its production, culminating in the B-17G, differing from its immediate predecessor by the addition of a chin turret with two caliber M2 Browning machine gun s under the nose. This eliminated the B-17's main defensive weakness in head-on attacks.

The Luftwaffe

After examining wrecked B-17s and B-24s, ''Luftwaffe '' officers discovered that on average it took around 20 hits with abbr=on}} shells fired from the rear to bring them down. Pilots of average ability hit the bombers with only about two percent of the rounds they fired, so to obtain 20 hits, the average pilot had to fire one thousand rounds at a bomber. Early versions of the Fw 190, one of the best German interceptor fighters, were equipped with two MG FF cannons, which carried only 500 rounds, and later with the better Mauser MG 151/20 cannons, which had a longer effective range than the MG FF weapon. The German fighters found that when attacking from the front, where fewer defensive guns were pointed, it only took four or five hits to bring a bomber down. To address the Fw 190's shortcomings, the number of cannons fitted was doubled to four with a corresponding increase in the amount of ammunition carried, and in 1944, a further upgrade to Rheinmetall -Borsig 's MK 108 cannon s was made, which could bring a bomber down in just a few hits.

The adoption of the ''Werfer-Granate 21'' (Wfr. Gr. 21) rocket mortar by the ''Luftwaffe'' in mid-August 1943 promised the introduction of a major "stand-off" style of offensive weapon–one strut-mounted tubular launcher was fixed under each wing panel on the ''Luftwaffe''s single-engined fighters, and two under each wing panel of a few twin-engined Bf 110 daylight ''Zerstörer'' aircraft. However, due to the ballistic drop of the fired rocket (despite the usual mounting of the launcher at about 15° upward orientation), and the small number of fighters fitted with the weapons, the Wfr. Gr. 21 never had a major effect on the combat box formations of Fortresses. Also, the attempts of the ''Luftwaffe'' to fit heavy-calibre ''Bordkanone''-series 37, 50 and even cannon as anti-bomber weapons on twin-engined aircraft such as the special Ju 88P fighters, as well as one model of the Me 410 ''Hornisse'', did not have much effect on the American strategic bomber offensive. The Me 262 had moderate success against the B-17 late in the war however. With its usual nose-mounted armament of four MK 108 cannon s, and with some examples later equipped with the R4M rocket, launched from underwing racks, it could fire from outside the range of the bombers' defensive guns and bring an aircraft down with one hit.

During World War II, after crash-landing or being forced down, approximately 4 B-17s were captured and refurbished by the ''Luftwaffe'' with about a dozen put back into the air. Given German markings, the captured B-17s were used to determine the airplane's vulnerabilities and to train German interceptor pilots in tactics. Others, with the cover designation Dornier Do 200, were used as long-range transports by the ''Kampfgeschwader'' 200 special duties unit, carrying out agent drops and supplying secret airstrips in the Middle East and North Africa. They were chosen for these missions as being more suitable for the role than available German aircraft and not in an attempt to deceive the Allies, being operated in full ''Luftwaffe'' markings. One of the B-17s of KG200, bearing ''Luftwaffe'' markings ''A3+FB'', was interned by Spain when it landed at Valencia airfield, 1944, and remained there for the rest of the war. Some B-17s kept their Allied markings and were used in attempts to infiltrate B-17 formations and report on their position and altitude. The practice was initially successful, but the Army Air Forces combat aircrews quickly developed and established standard procedures to first warn off, and then fire upon any "stranger" trying to join a group's formation.

Soviet use

The U.S. did not offer B-17s to the Soviet Union, however, at least 73 were used by the Soviet Air Force. These were aircraft that landed with mechanical trouble during the shuttle bombing raids over Germany, or had been damaged by a ''Luftwaffe'' raid in Poltava. The Soviets restored 23 to flying condition and concentrated them in the 890th bomber regiment of the 45th bomber division but they never saw combat. In 1946 the regiment was assigned to the Kazan factory to aid in the Soviet effort to reproduce the Boeing B-29 as the Tupolev Tu-4.

Postwar history

U.S. Air Force

Following the end of World War II, the B-17 was quickly phased out of use as a bomber and the Army Air Forces retired most of its fleet. Flight crews ferried the bombers back across the Atlantic to the United States where the majority were sold for scrap and melted down, although significant numbers remained in use in second-line roles such as VIP transports, air-sea rescue and photo-reconnaissance.Strategic Air Command (SAC), established in 1946, used reconnaissance B-17s (at first called F-9 [''F'' for ''Fotorecon''], later RB-17) until 1949. The USAF Air Rescue Service of the Military Air Transport Service (MATS) also operated B-17s as so-called "Dumbo" air-sea rescue aircraft. Work on using B-17s to carry airborne lifeboat s had begun in 1943, but they only entered service in the European theater in February 1945, also being used to provide search and rescue support for B-29 raids against Japan. About 130 B-17s were converted to the air-sea rescue role, at first designated B-17H and later SB-17G. Some had their defensive guns removed, while others retained their guns to allow use close to combat areas. The SB-17 served through the Korean War, remaining in service until the mid-1950s.

In 1946, surplus B-17s were chosen as drone aircraft for atmospheric sampling during the Operation Crossroads atomic bomb tests, being able to fly close to or even though the mushroom cloud s without endangering a crew. This led to more widespread conversion of B-17s as drones and drone control aircraft, both for further use in atomic testing and as targets for testing surface-to-air and air-to-air missile s. were converted to drones. The last operational mission flown by a USAF Fortress was conducted on 1959, when DB-17P ''44-83684'' directed QB-17G ''44-83717'' out of Holloman Air Force Base, New Mexico, as a target for an AIM-4 Falcon air-to-air missile fired from an F-101 Voodoo . A retirement ceremony was held several days later at Holloman, after which ''44-83684'' was retired to the Military Aircraft Storage and Disposition Center (MASDC) at Davis-Monthan Air Force Base, Arizona. Perhaps the most famous B-17, the ''Memphis Belle '', is being fastidiously restored to its wartime appearance by the National Museum of the United States Air Force.

U.S. Navy and U.S. Coast Guard

During the last year of the war and shortly thereafter, the United States Navy acquired 48 ex-USAAF B-17s for patrol and air-sea rescue work. The first two ex-USAAF B-17s, a B-17F (later modified to B-17G standard) and a B-17G were obtained by the Navy for various development programs. At first, these aircraft operated under their original USAAF designations but on July 31, 1945, they were assigned the naval aircraft designation PB-1, a designation which had originally been used in 1925 for the Boeing Model 50 experimental flying boat.

Thirty-two B-17Gs were used by the Navy under the designation PB-1W, the suffix -W standing for antisubmarine warfare. A large radome for a S-band AN/APS-20 search radar was fitted underneath the fuselage and additional internal fuel tanks were added for longer range, with the provision for additional underwing fuel tanks, while no armament was fitted. These aircraft were painted dark blue, a standard Navy paint scheme which had been adopted in late 1944. The PB-1W eventually evolved into an early warning aircraft by virtue of its APS-20 search radar. PB-1Ws continued in USN service until 1955, gradually being phased out in favor of the Lockheed WV-2 (known in the USAF as the EC-121 ), a military version of the Lockheed 1049 Constellation commercial airliner.

In July 1945, 16 B-17s were transferred to the Coast Guard via the Navy; these aircraft were initially assigned Navy Bureau numbers, but were delivered to the Coast Guard designated as PB-1Gs beginning in July 1946. Coast Guard PB-1Gs were stationed at a number of bases in the U.S. and Newfoundland, with five at Coast Guard Air Station Elizabeth City, North Carolina, two at CGAS San Francisco, two at NAS Argentia, Newfoundland, one at CGAS Kodiak, Alaska, and one in Washington state. They were used primarily for air-sea rescue, but were also used for iceberg patrol duties and for photo mapping. Air-sea rescue PB-1Gs usually carried a droppable lifeboat underneath the fuselage and the chin turret was often replaced by a radome. The Coast Guard PB-1Gs served throughout the 1950s, the last example not being withdrawn from service until October 14, 1959.

Variants/design stages

The B-17 went through several alterations in each of its design stages and variants. Of the 13 YB-17s ordered for service testing, 12 were used by the 2nd Bomb Group of Langley Field, Virginia, to develop heavy bombing techniques, and the 13th was used for flight testing at the Material Division at Wright Field, Ohio. Experiments on this aircraft led to the use of a turbo-supercharger which would become standard on the B-17 line. A 14th aircraft, the YB-17A, originally destined for ground testing only and upgraded with the turbocharger, was re-designated B-17A after testing had finished.

As the production line developed, Boeing engineers continued to improve upon the basic design. To enhance performance at slower speeds, the B-17B was altered to include larger rudder and flaps. The B-17C changed from gun blisters to flush, oval-shaped windows. Models ''A'' through ''D'' of the B-17 were designed defensively, while the B-17E was the first model primarily focused on offensive warfare. The B-17E was an extensive revision of the design; the fuselage was extended by, a much larger vertical fin and rudder were incorporated into the original design, and a gunner's position in the tail as well as an improved nose were added, resulting in a 20% increase in aircraft weight. The engines were upgraded to more powerful versions multiple times throughout its production, and similarly, the gun stations were altered on numerous occasions to enhance their effectiveness.

By the time the definitive B-17G appeared, the number of guns had been increased from seven to 13, the designs of the gun stations were finalized, and other adjustments were complete. The B-17G was the final version of the B-17, incorporating all changes made to its predecessor, the B-17F, and in total 8,680 were built, the last one (by Lockheed) on 1945. Many B-17Gs were converted for other missions such as cargo hauling, engine testing and reconnaissance. Initially designated SB-17G, a number of B-17Gs were also converted for search-and-rescue duties, later to be redesignated B-17H.

Two versions of the B-17 were flown under different designations, the XB-38 Flying Fortress and the YB-40 Flying Fortress. The XB-38 was an engine test-bed for Allison V-1710 liquid-cooled engines, should the Wright engines normally used on the B-17 become unavailable. The only prototype XB-38 to fly crashed on its ninth flight and the type was abandoned, the V-1710 being kept for fighters. The YB-40 was a heavily armed modification of the standard B-17 used before the P-51 Mustang, an effective long-range fighter, became available to act as escort. Additional armament included an additional dorsal turret in the radio room, a chin turret (which became standard with the B-17G) and twin guns in the waist positions. The ammunition load was over 11,000 rounds, making the YB-40 well over heavier than a fully loaded B-17F. The YB-40s with their numerous heavy modifications had trouble keeping up with the lighter bombers once they had dropped their bombs, and so the project was abandoned and finally phased out in July 1943.

Late in World War II, at least 25 B-17s were fitted with radio controls and television cameras, loaded with of high-explosives and dubbed BQ-7 "Aphrodite missiles" for Operation Aphrodite . The operation, which involved remotely flying Aphrodite drones onto their targets by accompanying CQ-17 "mothership" control aircraft, was approved on, 1944, and assigned the 388th Bombardment Group stationed at RAF Fersfield, a satellite of RAF Knettishall. The first four drones were sent to Mimoyecques, Siracourt, Watten and Wizernes on August 4, causing little damage. The project came to a sudden end with the unexplained mid-air explosion over the Blyth estuary of a B-24 Liberator, part of the United States Navy 's contribution as "Project Anvil", en route for Heligoland piloted by Lieutenant Joseph P. Kennedy Jr., future U.S. president John F. Kennedy 's elder brother. British authorities were anxious that no similar accidents should again occur, and the Aphrodite project was scrapped in early 1945.

During and after World War II, a number of weapons were tested and used operationally on B-17s. Some of these weapons included "razons" (radio-guided) glide bombs, and Republic-Ford JB-2 s, also nicknamed "Thunderbugs" (American reverse-engineered models of the German V-1 Buzz Bomb). A much-used traveling airborne shot of a V-1/JB-2 launch in World War II documentaries was filmed from a USAF A-26 of the Air Proving Grounds, Eglin Air Force Base, launched from Santa Rosa Island, Florida.

Operators

The B-17 was a versatile aircraft, serving in dozens of USAAF units in theaters of combat throughout World War II, and in non-bomber roles for the RAF. Its main use was in Europe, where its shorter range and smaller bombload relative to other aircraft available did not hamper it as much as in the Pacific Theater. Peak USAAF inventory (in August 1944) was 4,574 worldwide.

Survivors

There are a total of 53 surviving airframes worldwide:

* 12 active flying

* 9 on static display

* 2 currently undergoing restoration to fly

* 3 currently undergoing restoration for display

* 5 in storage

* 19 partial airframes/hulks

Fortresses as a symbol

The B-17 Flying Fortress has become, for many reasons, an icon of American power and a symbol of its Air Force. During the 1930s, the USAAC, as articulated by then-Major General Frank Maxwell Andrews and the Air Corps Tactical School, touted the bomber as a strategic weapon. General Henry H. Arnold, Chief of the Air Corps, recommended the development of bigger aircraft with better performance and the Tactical School agreed completely.

When the Model 299 was rolled out on 1935, bristling with multiple machine gun installations, Richard Williams, a reporter for the ''Seattle Times '' coined the name "Flying Fortress" with his comment "Why, it's a flying fortress!". Boeing was quick to see the value of the name and had it trademarked for use. In 1943, Consolidated Aircraft commissioned a poll to see "to what degree the public is familiar with the names of the Liberator and the Flying Fortress." Of 2,500 men in cities where Consolidated adverts had been run in newspapers, 73% had heard of the B-24 Liberator, while 90% knew of the B-17.

After the initial B-17s were delivered to the Air Corps 2nd Bombardment Group, they were used on promotional flights emphasizing its great range and navigational precision. In January 1938, group commander Colonel Robert Olds flew a YB-17 from the east to west coast, setting a transcontinental record of 13 hours 27 minutes. He also broke the west-to-east coast record on the return trip, averaging in 11 hours 1 minute. Six bombers of the 2nd Bombardment group took off from Langley Field on 1938 as part of a goodwill flight to Buenos Aires, Argentina . Covering they returned on, with seven aircraft setting off on a flight to Rio de Janeiro, Brazil, three days later. In a well-publicized mission on May 12 of the same year, three B-17s "intercepted" and took photographs of the Italian ocean liner SS ''Rex'' off the Atlantic coast.

During the war, the largest offensive bombing force, the Eighth Air Force, had an open preference for the B-17. Lieutenant General Jimmy Doolittle wrote about his preference for equipping the Eighth with B-17s, citing the logistical advantage in keeping fielded forces down to a minimum number of aircraft types with their unique servicing and spares. For this reason, he wanted B-17 bombers and P-51 fighters for the Eighth. His views were supported by Eighth Air Force statisticians, whose studies purportedly showed that Fortresses had utility and survivability much greater than that of the B-24. Making it back to base on multiple occasions despite extensive battle damage, its durability took on mythical proportions; stories and photos of B-17s surviving battle damage were widely circulated during the war. Despite an inferior performance and bombload compared to the more numerous B-24 Liberator, a survey of Eighth Air Force crews showed a much higher rate of satisfaction in the B-17.

Hollywood featured the airplane in its movies, such as ''Twelve O'Clock High '' starring Gregory Peck . This film was made with the full cooperation of the United States Air Force and made use of actual combat footage. In 1964, the movie was made into a television show of the same name and ran for three years. The B-17 also appeared in the 1938 movie ''Test Pilot '' with Clark Gable and Spencer Tracy, with Clark Gable in ''Command Decision'' in 1948, in ''Tora! Tora! Tora! '' in 1970, and in ''Memphis Belle'' with Matthew Modine, Eric Stoltz, Billy Zane, and Harry Connick, Jr. in 1990. The most famous B-17, the ''Memphis Belle'', toured the U.S. with its crew to reinforce national morale (and to sell War Bonds ), and starred in a USAAF documentary, ''Memphis Belle: A Story of a Flying Fortress ''.

Notable B-17s

* Aluminum Overcast -- flying example

*Yankee Lady -- flying example Yankee Air Force

* Liberty Belle – former engine testbed restored as flying example

* Memphis Belle

* My Gal Sal

* Murder Inc. — A B-17 bombardier wearing the name of the B-17 "Murder Inc." on his jacket was used for propaganda in German newspapers

* Nine-O-Nine

* Old 666

* Piccadilly Lilly II

* (The) Pink Lady

* Sally B – flying example

* Sentimental Journey

* Shoo Shoo Baby

* Swamp Ghost

* (The) Swoose

* Texas Raiders -- flying example Commemorative Air Force

* Thunderbird

* Ye Olde Pub – the B-17 that Franz Stigler did not shoot down, as memorialized in "''A Higher Call''" by John D. Shaw

Noted B-17 pilots and crew members

Medal of Honor awards

Many B-17 crew members received military honors and 17 received the Medal of Honor, the highest military decoration awarded by the United States:

* Brigadier General Frederick Castle (flying as co-pilot) - awarded posthumously for remaining at controls so others could escape damaged aircraft.

* 2nd Lt Robert Femoyer (navigator) - awarded posthumously

* 1st Lt Donald J. Gott (pilot) - awarded posthumously

* 2nd Lt David R. Kingsley (bombardier) - awarded posthumously for tending to injured crew and giving up his parachute to another

* 1st Lt William R. Lawley, Jr. - "heroism and exceptional flying skill"

* Sgt Archibald Mathies (engineer-gunner) - awarded posthumously

* 1st Lt Jack W. Mathis (bombardier) - posthumously, the first airman in the European theater to be awarded the Medal of Honor

* 2nd Lt William E. Metzger, Jr. (Co-pilot) - awarded posthumously

* 1st Lt Edward Michael

* 1st Lt John C. Morgan

* Capt Harl Pease (awarded posthumously)

* 2nd Lt Joseph Sarnoski (awarded posthumously)

* S/Sgt Maynard H. Smith (gunner)

* 1st Lt Walter E. Truemper (awarded posthumously)

* S/Sgt Forrest L. Vosler (radio operator)

* Brig Gen Kenneth Walker (not part of crew at time) - awarded posthumously

* Maj Jay Zeamer, Jr. (pilot) - earned on unescorted reconnaissance mission

Other military achievements or events

* Allison C. Brooks (1917–2006): Was awarded numerous military decorations, and was ultimately promoted to the rank of Major General and served in active duty until 1971.

* 1st Lt Emil "Mickey" Cohen (1924–2008): Nicknamed "The Kid". Flew with the 447th Bombardment Group, 709th squadron, out of Rattlesden, England. Piloted the Barbara Jane and two missions on The Blue Hen Chick. Was the youngest B-17 pilot in the 8th Air Force and may have been the youngest bomber pilot in the U.S. Army Air Forces.

* 1st Lt Eugene Emond (1921–1998): Lead Pilot for Man O War II Horsepower Limited received the Distinguished Flying Cross, Air Medal with three Oak Leaf Clusters, American Theater Ribbon and Victory Ribbon. Was part of D-Day and witnessed one of the first German jets when a ME-262 flew through his formation over Germany: one of the youngest bomber pilots in the U.S. Army Air Forces.

* Captain Werner G. Goering : American-born nephew of the Nazi Commander of the ''Luftwaffe'' in World War II, Hermann Göring .

* Immanuel J. Klette (1918–1988): Second-generation German-American whose 91 combat missions were the most flown by any Eighth Air Force pilot in World War II.

* Colin Kelly (1915–1941): Pilot of the first U.S. B-17 lost in action.

* Col Frank Kurtz (1911–1996): The USAAF's most decorated pilot of World War II; Commander of the 463rd Bombardment Group (Heavy), 15th Air Force, Celone Field, Foggia, Italy; Clark Field Philippines attack survivor; Olympic bronze medalist in diving (1932), 1944–1945; father of actress Swoosie Kurtz .

* Gen Curtis LeMay (1906–1990): Became head of the Strategic Air Command and head of the USAF.

* Lt Col Nancy Love (1914–1976) and Betty (Huyler) Gillies (1908–1998): The first women to be certified to fly the B-17, in 1943.

* Col Robert K. Morgan (1918–2004): Pilot of ''Memphis Belle''.

* Lt Col Robert Rosenthal (1917–2007): Commanded the only surviving B-17, "Rosie's Riveters", of a US 8th Air Force raid by the 100th Bomb Group on Münster in 1943, earned sixteen medals for gallantry (including one each from Britain and France), and led the raid on Berlin on February 3, 1945, that is likely to have ended the life of Roland Freisler, the Third Reich 's infamous "hanging judge".

* Brig Gen Paul Tibbets (1915–2007): Flew with the 97th Bombardment Group (Heavy) with both the 8th Air Force in England and the 12th Air Force in North Africa; later pilot of the B-29 Enola Gay dropping the atomic bomb on Hiroshima, Japan.

* Robert Webb (1922–2002): One of the youngest bomber pilots in the U.S. Army Air Forces, received the Distinguished Flying Cross with seven Oak Leaf Clusters.

Civilian achievements or events

* Martin Caidin (1927–1997): Author of ''Cyborg '', the story that formed the basis of ''The Six Million Dollar Man '' wrote the saga of the last transatlantic formation flight of B-17s ever made, ''Everything But the Flak''.

* Clark Gable (1901–1960): Academy Award -winning film actor, five missions as waist gunner with several groups from May to September 1943, including the B-17 ''Eight Ball'' of the 359th Bomb Squadron (351st Bomb Group).

* Tom Landry (1924–2000): American football player and coach, flew 30 missions over Europe in 1944–45 as a B-17 pilot with the 493rd Bomb Group, surviving a crash landing in Czechoslovakia. (His older brother Robert died in a B-17 crash)

* Norman Lear : Radio operator, with the 463rd Bombardment Group (Heavy), 15th Air Force, Celone Field, Foggia, Italy; television producer of American sitcoms ''Sanford and Son '', ''Maude '' and ''All in the Family '', among others.

* Gene Roddenberry (1921–1991): Creator of ''Star Trek ''; flew B-17s for the 394th Bomb Squadron, 5th Bomb Group (H), in the Pacific theater.

* Robert Rosenthal (1917–2007): Assistant to the U.S. prosecutor at the Nuremberg Trials, where he interrogated Hermann Göring, pilot with the 100th Bomb Group.

* Brigadier General Robert Lee Scott, Jr. (1908–2006): Best known for his autobiography ''God is My Co-Pilot'', about his exploits in World War II with the Flying Tigers and the United States Army Air Forces in China and Burma.

* James Stewart (1908–1997): Academy Award-winning film actor, instructed in B-17s before flying 20 combat missions in B-24s with the 8th Air Force, England; retired from Air Force Reserve as a Brigadier General.

* Bert Stiles (1920–1944): 91st Bomb Group co-pilot from March to October 1944, short-story author, killed in action flying a P-51 on a second tour.

* Bruce Sundlun (1920– ): 384th Bomb Group Pilot of B-17F ''Damn Yankee'' avoided capture after being shot down over Jabbeke, Belgium, 1943 to become a lawyer, businessman and Governor of Rhode Island 1991–95.

* Smokey Yunick (1923–2001): Award-winning motorsports car designer and premier NASCAR crew chief flew 50 missions as a B-17 pilot with the 97th Bombardment Group (Heavy) of the 15th Air Force, out of Amendola Airfield, Foggia, Italy.