In 1934 the United States Army issued a specification for a long-range high-altitude daylight bomber for its Air Corps. The Boeing Aircraft Company responded with a prototype, the Boeing model 299, powered by four 750-hp Pratt and Whitney Hornet engines. This machine made its maiden flight on the 28th July 1935, but was unfortunately destroyed in an accident in its early trials. Nevertheless the project continued with a further fourteen machines (designated YP-17 or B-17A) and after expensive testing the first batch of 39 production machines, designated B-17B, were delivered to the Air Corps in March 1940.
Expansion of the Air Corps would lead to a further order for 38 B-17Cs powered by the more powerful 1200-hp Wright Cyclone engine. Twenty of these would be transferred to Great Britain in 1941 for evaluation by the Royal Air Force under combat conditions. The Fortress Mk.I, as it was known in British service, failed however to attract much interest –a number being lost during high-altitude bombing raids not just as a result of enemy action but also to mechanical failures. The surviving aircraft being soon relegated to RAF Coastal Command for Maritime patrol missions.
The United States Air Corps however maintained their belief in precision daylight bombing raids for which the extensive armament of the B-17, consisting of upper and lower turrets and guns in the nose and tail, would provide sufficient defence against enemy fighter aircraft. By the time that the United States entered the War in December 1941 the B-17 had already developed through the B-17D to the B-17E which had a redesigned rear fuselage incorporating waist and ball turret gun positions plus an enlarged fin with a prominent fillet. It would be the B-17E which would be deployed to bases in the United Kingdom with the US 8TH Air Force during 1943 contributing to the Allied Air Offensive against German cities. A total of 512 B-17E and from April 1942 3,400 B-17F (nineteen of which would go to the RAF Coastal Command as the Fortress Mk.II) would be built.
The most numerous version of the Fortress was the B-17G (Fortress B.Mk.III in RAF Service), which began equipping US Bombardment groups from the autumn of 1943. 8,680 of these were built by Boeing, Lockheed-Vega and Douglas. The B-17G had improved flight controls, a chin nose turret, and new engine turbochargers.
After the war surplus B-17s found new roles in such tasks as civil freighting, aerial survey and fire bombing, whilst a few were sold onto foreign governments (mainly in South America). A few saw further service in the Korean War serving as mainly in the less glamorous roles of air-rescue and Staff transport, and at least two were used by the CIA on covert operations over South East Asia in the 1950s. About six B-17s remain airworthy as display warbirds.
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