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The Boeing B-17 Flying Fortress is a four-engine heavy bomberdeveloped 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 exceeded the air corps' performance specifications. Although Boeing lost the contract because the prototype crashed, the air corps was so impressed with Boeing's design that it 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 many airfields in central and southern England, and the Fifteenth Air Force, based in Italy, complemented the RAF Bomber Command's nighttime area bombing in the Combined Bomber Offensive to help secure air superiority over the cities, factories and battlefields of Western Europe in preparation for the invasion of France in 1944. The B-17 also participated to a lesser extent in the War in the Pacific, early in World War II, where it conducted raids against Japanese shipping and airfields.
From its prewar inception, the USAAC (by June of 1941, the USAAF) promoted the aircraft as a strategic weapon; it was a fast, high-flying, long-range bomber that was able to defend itself, carried a very good bombload and was able to return home despite extensive battle damage. Its reputation quickly took on mythic proportions, and widely circulated stories and photos of notable numbers and examples of B-17s surviving battle damage increased its iconic status. With aservice 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 1.5 million tonnes of bombs dropped on Germanyand its occupied territories by U.S. aircraft, 640,000 tonnes were dropped from B-17s. In addition to its role as a bomber, the B-17 was also employed as a transport, antisubmarine warfare platform, drone controller, and search-and-rescue aircraft.
As of May 2015, ten aircraft remain airworthy. None of them are combat veterans. Additionally, a few dozen more are in storage or on static display. The oldest is a D-series combat veteran with service in the Pacific and the Caribbean.
On 8 August 1934, the U.S. Army Air Corps (USAAC) tendered a proposal for a multi-engine bomber to replace the Martin B-10. The air corps was 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 10,000 feet (3 km) for ten hours with a top speed of at least 200 miles per hour (320 km/h).
They also desired, but did not require, a range of 2,000 miles (3,200 km) and a speed of 250 miles per hour (400 km/h). 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, with the Boeing factory designation of Model 299, was designed by a team of engineers led by E. Gifford Emery and Edward Curtis Wells, and was built at Boeing's own expense. It combined features of the experimental Boeing XB-15 bomber with the Boeing 247 transport aircraft. The B-17's armament consisted of up to 4,800 pounds (2,200 kg) of bombs on two racks in the bomb bay behind the cockpit, and initially possessed five 0.30 inches (7.62 mm) machine guns. It was powered by four Pratt & Whitney R-1690"Hornet" radial engines each producing 750 horsepower (600 kW) at 7,000 feet (2,100 m).
The first flight of the Model 299 was on 28 July 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. The most unusual gun emplacement was the nose installation which allowed the single machine gun to be fired toward almost any frontal angle.
Boeing was quick to see the value of the name and had it trademarked for use. Boeing also claimed in some of the early press releases that Model 299 was the first combat aircraft that could continue its mission if one of its four engines failed. On 20 August 1935, the prototype flew from Seattle to Wright Field in nine hours and three minutes at an average cruising speed of 252 miles per hour (406 km/h), 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. Major General Frank Maxwell Andrews of theGHQ Air Force believed that the long-range capabilities of four-engine large aircraft were more effective than shorter-ranged, twin-engine aircraft, and that the B-17 was better suited to new, emerging USAAC 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 30 October 1935, Army Air Corps test-pilot Major Ployer Peter Hill and Boeing employee Les Tower took the Model 299 on a second evaluation flight. The crew forgot to disengage the "gust locks", a system of devices integral to the design that held the bomber's movable control surfaces in place while the aircraft was parked on the ground. After takeoff, due to the failure to manually disengage all of the gust locks, the aircraft entered a steep climb, stalled, nosed over, and crashed, killing Hill and Tower (other observers survived with injuries).[N 1]
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 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.
The loss was not total... but Boeing's hopes for a substantial bomber contract were dashed.— Peter Bowers, 1976
Regardless, the USAAC had been impressed by the prototype's performance, and on 17 January 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 pre-flight checklist to avoid accidents such as that which befell the Model 299.[N 2] 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 610 miles (980 km) 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 29 April 1938. The aircraft was delivered to the army on 31 January 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, ten more aircraft designated B-17B were ordered to equip two bombardment groups, one on each U.S. coast. Improved with larger flaps, rudder and a well-framed, ten-panel plexiglas nose, the B-17Bs were delivered in five small batches between July 1939 and March 1940. In July 1940, an 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 11 January 1937 and 30 November 1941, but production quickly accelerated, with the B-17 eventually achieving the highest production rate for any large aircraft.[N 3] 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).
Though the crash of the prototype 299 in 1935 had almost wiped out Boeing, now it was seen as a boon. Instead of building models based on experimental engineering, Boeing had been hard at work developing their bomber and now had versions ready for production far better than would have been possible otherwise. One of the most significant weapons of World War II would be ready, but only by a hair.— Jeff Ethell, 1985
|Model 299||1||28 July 1935|
|YB-17||13||2 December 1936|
|YB-17A||1||29 April 1938.|
|B-17B||39||27 June 1939|
|B-17C||38||21 July 1940|
|B-17D||42||3 February 1941|
|B-17E||512||5 September 1941|
|B-17F (total)||3,405||30 May 1942|
|B-17G (total)||8,680||August 16, 1943|
B-17s were built at Boeing Plant 2
Seattle, Washington (BO)
and starting with the B-17F also at
Lockheed Vega, Burbank California (VE) and
Douglas Aircraft, Long Beach California (DL)
The aircraft 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 quartet of turbo-superchargers 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 turbochargers, 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 rudders and flaps. The B-17C changed from three bulged, oval shaped machine gun blisters to two flush, oval-shaped machine gun window openings and a single "bathtub" machine gungondola housing on the lower fuselage, that resembled the similarly-configured and located ventral defensive emplacement on the German Heinkel He 111P-series medium bomber. Models A through D of the B-17 were designed defensively, while the large-tailed B-17E was the first model primarily focused on offensive warfare.
The B-17E was an extensive revision of the Model 299 design: The fuselage was extended by 10 ft (3.0 m); a much larger rear fuselage, vertical tailfin, rudder, and horizontal stabilizer were added to the design; a gunner's position was added in the new tail; the nose (especially the bombardier's well-framed nose glazing) remained relatively the same as the earlier B through D versions had, but with the addition of a Sperry electrically-powered manned dorsal gun turret just behind the cockpit, and the similarly-powered (also built by Sperry) manned ventral ball turret just aft of the bomb bay – replacing a relatively hard-to-use, Bendix-designed remotely-operated ventral turret on the earliest examples of the E variant, that had also been used on the earlier marks of the North American B-25 Mitchell – resulted in a 20% increase in aircraft weight. The B-17's turbocharged Wright R-1820 Cyclone 9 engines were upgraded to increasingly more powerful versions of the same powerplants multiple times throughout its production, and similarly, the number of machine gun emplacement locations was increased to enhance their aircraft's combat effectiveness.
The B-17F variants were the primary versions flying for the Eighth Air Force to face the Germans in 1943, and had standardized the manned Sperry ball turret for ventral defense, replacing the earlier, ten-panel well-framed bombardier's nose glazing from the B subtype with an enlarged, nearly frameless plexiglas bombardier's nose enclosure for improved forward vision.
Two experimental versions of the B-17 were flown under different designations, theXB-38 Flying Fortress and the YB-40 Flying Fortress. The XB-38 was an engine testbed 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 theNorth American 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 remotely operated and fired Bendix-built "chin turret" and twin .50 in (12.7 mm) guns in each of the waist positions. The ammunition load was over 11,000 rounds. All of these modifications made the YB-40 well over 10,000 lb (4,500 kg) 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, but not before the final production blocks of the B-17F from Douglas' plants adopted the YB-40's remotely operated and fired Bendix "chin turret" for a much-improved forward defensive weapons installation.
By the time the definitive B-17G appeared, the number of guns had been increased from seven to thirteen, the designs of the gun stations were finalized, and other adjustments were completed. The B-17G was the final version of the Flying Fortress, incorporating all changes made to its predecessor, the B-17F, adopting the remotely operated "chin turret" for forward defense from the YB-40 "gunship" version, and in total 8,680 were built, the last one (by Lockheed) on 28 July 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
Late in World War II, at least 25 B-17s were fitted with radio controls and television cameras, loaded with 20,000 lb (9,100 kg) 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 26 June 1944, and assigned to the 388th Bombardment Group stationed atRAF Fersfield, a satellite of RAF Knettishall.
The first four drones were sent to Mimoyecques, the Siracourt V-1 bunker, Wattenand Wizernes on 4 August, causing little damage. The project came to a sudden end with the unexplained mid-air explosion over the Blyth estuary of a B-24, part of the United States Navy's contribution as "Project Anvil", en route for Heligolandpiloted by Lieutenant Joseph P. Kennedy Jr., future U.S. president John F. Kennedy's elder brother. Blast damage was caused over a radius of 5 miles (8.0 km). British authorities were anxious that no similar accidents should again occur, and the Aphrodite project was scrapped in early 1945.
The B-17 began operations in World War II with the Royal Air Force (RAF) in 1941 (but was not successful), and in the Southwest Pacific with the U.S. Army. The 19th Bombardment Group had deployed to Clark Field in the Philippines a few weeks before the Japanese attack on Pearl Harbor as the first of a planned heavy bomber buildup in the Pacific. Half of the group's B-17s were wiped out on 8 December 1941 when they were caught on the ground during refueling and rearming for a planned attack on Japanese airfields on Formosa. The small force of B-17s operated against the Japanese invasion force until they were withdrawn to Darwin, in Australia's Northern Territory. In early 1942, the 7th Bombardment Group began arriving in Java with a mixed force of B-17s and LB-30/B-24s. A squadron of B-17 from this force detached to the Middle East to join the First Provisional Bombardment Group thus becoming the first American B-17 squadron to go to war against the Germans. After the defeat in Java, the 19th withdrew to Australia where it continued in combat until it was sent back home by General George C. Kenney when he arrived in Australia in mid-1942. In July 1942, the first USAAF B-17s were sent to England to join the Eighth Air Force. Later that year two groups moved to Algeria to join Twelfth Air Force for operations in North Africa. The B-17s were primarily involved in the daylight precision strategic bombing campaign against German targets ranging from U-boat pens, docks, warehouses and airfields to industrial targets such as aircraft factories. In the campaign against German aircraft forces in preparation for the invasion of France, B-17 and B-24 raids were directed against German aircraft production while their presence drew the Luftwafffighters 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 640,036 short tons (580,631 metric tons) of bombs on European targets (compared to 452,508 short tons (410,508 metric tons) dropped by the Liberator and 463,544 short tons (420,520 metric tons) dropped by all other U.S. aircraft). The British heavy bombers, the Avro Lancaster and Handley Page Halifax, dropped 608,612 long tons (681,645 short tons) and 224,207 long tons (251,112 short tons) respectively.
The Royal Air Force entered World War II with no heavy bomber of its own in service; the biggest available were long-range medium bombers such as theVickers Wellington which could carry up to 4,500 pounds (2,000 kg) of bombs.While the Short Stirling and Handley Page 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 8 July 1941 was unsuccessful; on 24 July, the target was the Scharnhorst, anchored in Brest. Considerable damage was inflicted on the vessel.
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 Commandabandoned daylight bombing raids using the Fortress I because of the aircraft'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 misgivings by the RAF that attempts at 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 (B-17F) and 45 Fortress Mk IIA (B-17E). A Fortress from No. 206 Squadron RAF sank U-627 on 27 October1942, the first of 11 U-boat kills credited to RAF Fortress bombers during the war.
The RAF's No. 223 Squadron, as part of 100 Group, operated a number of Fortresses equipped with an electronic warfare system known as "Airborne Cigar" (ABC). This was operated by German–speaking radio operators who would identify and jam German ground controllers' broadcasts to their nightfighters. They could also pose as ground controllers themselves with the intention of steering nightfighters away from the bomber streams.
The air corps (renamed United States Army Air Forces (USAAF) on 20 June 1941), using the B-17 and other bombers, bombed from high altitudes using the then-secret Norden bombsight, known as the "Blue Ox", 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 12 May 1942, to form the 97th Bomb Group. On 17 August 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 close escorted by four squadrons of RAF Spitfire IXs (and a further five squadrons of Spitfire Vs to cover the withdrawal) on the first USAAF heavy bomber raid over Europe, against the large railroad marshalling yards at Rouen-Sotteville in France, while a further six aircraft flew a diversionary raid along the French coast. The operation, carried out in good visibility, was a success, with only minor damage unrelated to the enemy to one aircraft and half the bombs landing in the target area. The raid helped assuage British doubts about the capabilities of American heavy bombers in operations over Europe.
Two additional groups arrived in England at the same time, bringing with them the first B-17Fs, which would serve as the primary AAF heavy bomber fighting the Germans until September 1943. As the raids of the American bombing campaign grew in numbers and frequency, German interception efforts grew in strength (such as during the attempted bombing of Kiel on 13 June 1943), such that unescorted bombing missions came to be discouraged.
The two different strategies of the American and British bomber commands were organized at the Casablanca Conferencein January 1943. The resulting "Combined Bomber Offensive" would weaken the Wehrmacht, destroy German morale and establish air superiority through Operation Pointblank's destruction of German fighter strength in preparation for a ground offensive. The USAAF bombers would attack by day, with British operations – chiefly against industrial cities – by night.
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