In 1934 the US Army Air Corps issued a specification for a multi-engined anti-shipping aircraft. A two-motor design was expected,  but Boeing came up with a design for a four-engined aircraft able to carry the same bomb load faster and higher. This aircraft had Pratt & Whitney Hornet engines (each producing 750 horsepower),  a crew of 8,  and internal stowage for eight 600-lb bombs.  Designated the '299' it first flew on 28 July 1935.

The first of the service-test batch of 13 YB-17s was delivered in January 1937,   powered by the Wright Cyclone engine.   All subsequent models were equipped with turbocharged engines giving  improved high-altitude performance. 

The first production model  -  the B-17B of June 1939 - had a different nose configuration, and a larger rudder and flaps.   The B-17C had more guns,  better armour, and self-sealing fuel tanks. 

Twenty of the B-17C were supplied to the UK.  90 Squadron of Bomber Command used them in a few high-level bombing missions over German-occupied Europe.  It was,  however,  found that the Norden bomb-sight tended to malfunction,  the Browning machine-guns to freeze at high altitude and that German fighters would attack from astern in a defensive blind spot.

The improved B17-D bore the brunt of the heavy bomber's early fighting in the Pacific.  Extensive combat experience led to the B-17E,  with powered ventral ("ball"), dorsal and tail turrets, a huge fin for high-level bombing accuracy, and a great increase in armour and other equipment.

The B-17E was the initial weapon of the the US Eighth Air Force,  and on 17 August 1942 opened the gruelling two-and-a-half year daylight bombing campaign against German industry.  However, the E was quickly displaced by the B-17F,  which had a clear plexiglas nose,  paddle-bladed props, and provision for underwing racks.   3,405 of the B-17F were built.