Pursuing an operational theory that high flying, heavily armed bombers were the surest means of striking strategic targets in daylight, the US Army Air Corps issued a requirement in 1934 for which the Boeing Model 299 Flying Fortress was designed and first flown on 28 July 1935. Twelve Y1B-17 (later B-17) service test aircraft entered service in 1937 and were followed by small numbers of B-17B and B-17C bombers in 1940-1, and by the B-17D in 1941. The B-17E introduced the enlarged vertical tail surfaces and tail gun position characteristic of all subsequent B-17s, as well as poweroperated twin-gun turrets aft of the cockpit and below the centre fuselage, 512 B-17Es were produced, this version being the first US Army Air Force heavy bomber to see combat in Europe with the 8th Air Force. A total of 3,400 B-17F bombers, with enlarged one-piece nose transparency, was produced during 1942-3, and these were followed by the principal variant, the B-17G, which, in reply to calls for improved nose armament to counter the Luftwaffe's head-on attacks, introduced the two-gun 'chin' turret; production totalled 8,680 B-17G aircraft by Boeing, Douglas and Lockheed-Vega.
escort fighters - the P-38, P-47 and P-51. One temporary expedient involved the use of a small number of B-17s modified as YB-40 'escort' aircraft, some aircraft carrying up to 30 machine-guns. Fortresses (B-17Cs, Fs and Gs) served in small numbers with RAF Bomber and Coastal Commands.
37 minutes; service ceiling 10850 m (35,600ft); range with 2722-kg (6,000-Ib) bombload 3220 km (2,000 miles)