The B-17 started life as a prototype designated 'Boeing Model 299', which flew for the first time on 28th July 1935. Fourteen more evaluation models, eventually known as the B17 and B17A lead to a modified production model, the B-17B, of which 39 were delivered by the end of March, 1940. In 1941, twenty further modified B-17Cs were delivered to the RAF, where they were known as the Fortress 1, and 42 similar B-17Ds were supplied for Pacific service.

The early B-17s did not fare very well. The prototype had been destroyed in an accident, although this was put down to human error; the RAF models were diverted to coastal command in the middle east after several were lost in bombing missions, and about half of the early model B-17s were destroyed in in the Far East.

The B-17E brought changes to the tail design and improvements in armament which included, the first time, a tail gunner. This was the first type to see active service in europe. The RAF's B-17Es were known as the Fortress11A, 42 of which were delivered in 1942 as part of a total of 512 of this variant. There were 3,400 of the next type, the B-17F, the last of which incorporated a pair of chin-mounted and power-operated 12.7mm guns. This was carried into the B-17G, the model that became the main production B-17, boasting up to 14 machine guns.

The B-17 was critical to the USAAF daylight precision bombing campaign. Flying in formation, the Fortresses battled through German defences in daylight raids but still suffered heavy casualties until the addition of long-range escort fighters, the P-47 Thunderbolt and P-51 Mustang, and the introduction of the 25 mission limit.

Combat crews lives were measured in weeks rather than months but, despite the high casualty rate, B-17's became famous for returning home with terrible battle damage for which they were fondly regarded by their crews.