In May 1934 the US Army Air Corps issued a specification for a multi-engined anti-shipping bomber to defend the nation against enemy fleets. The answer was expected to be similar to the Martin B-10, but Boeing proposed four engines in order to carry the same bomb load faster and higher. It was a huge financial risk for the Seattle company, but the resulting Model 299 was a giant among combat aircraft, with four Pratt & Whitney Hornet engines, a crew of eight and stowage for eight 600 lb (272 kg) bombs internally. On delivery to Wright Field it crossed the nation at 232 mph (374 km/h), causing editors to think of the Flying Fortress in a new light. With the service-test batch of 13 Y1B-17 the Wright Cyclone engine was adopted, later versions all being turbocharged for good high-altitude performance. The production B-17B introduced a new nose and bigger rudder and flaps, though the wing loading was conservative and an enduring characteristic of every "Fort" was sedate flying.
With the B-17C came a ventral bathtub, flush side guns, armour and self-sealing tanks. In return for combat data 20 were supplied to the RAF, which used them on a few high altitude daylight raids with 90 Squadron of Bomber Command. It was found that the Norden sight tended to malfunction, the Browning guns to freeze at the high altitude and German fighters to attack from astern in a defensive blind spot. While surviving Fortress 1's operated with coastal and Middle East forces, the improved B-17D joined the US Army and bore the brunt of early fighting in the Pacific. But extensive combat experience led to the redesigned B-17E, with powered dorsal, ventral (ball) and tail turrets, a huge fin for high-altitude bombing accuracy and much more armour and equipment. This went into huge production by Boeing, Lockheed-Vega and Douglas-Tulsa. It was the first weapon of the US 8th Bomber Command in England, and on 17 August 1942 began three gruelling years of day strategic bombing in Europe.
Soon the E gave way to the B-17F, of which 3,405 were built, with many detail improvements, including a long Plexiglass nose, paddle-blade propellors and provision for underwing racks. At the end of 1942 came the final bomber model, the B-17G, with chin turret and flush staggered waist guns. A total of 8,680 G models were built, Boeing's Seattle plant alone turning out 16 a day, and the total B-17 run amounted to 12,731. A few B-17F's were converted to XB-40's, carrying extra defensive guns to help protect the main Bomb Groups, while at least 25 were turned into BQ-7 Aphrodite radio-controlled missiled loaded with 12,000 lb (5,454 kg) of high explosive for use against the U-Boat shelters. Many F and G models were fitted with H2X radar with the scanner retracting into the nose or rear fuselage, while other versions included the F-9 reconnaisance, XC-108 executive transport, CB-17 utility transport, PB-1W radar early-warning, PB-1G lifeboat-carrying air/sea rescue and QB-17 target drone. After the war came other photo, training, drone-director, search/rescue and research versions, including many used as engine and equipment testbeds.