The B-17 went through several alterations in each of its design stages and variants. Of the 13 YB-17s ordered for service testing, 12 were used by the 2nd Bomb Group of Langley Field, Virginia, to develop heavy bombing techniques, and the 13th was used for flight testing at the Material Division at Wright Field, Ohio. Experiments on this aircraft led to the use of a turbo-supercharger which would become standard on the B-17 line. A 14th aircraft, the YB-17A, originally destined for ground testing only and upgraded with the turbocharger, was re-designated B-17A after testing had finished. As the production line developed, Boeing engineers continued to improve upon the basic design. To enhance performance at slower speeds, the B-17B was altered to include larger rudder and flaps. The B-17C changed from three bulged, oval shaped machine gun blisters to two flush, oval-shaped machine gun window openings and a single “bathtub” machine gun housing on the lower fuselage, that resembled the similarly-configured and located ventral defensive emplacement on the German Heinkel He 111P-series medium bomber. Models A through D of the B-17 were designed defensively, while the large-tailed B-17E was the first model primarily focused on offensive warfare.
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