The B-17B (299M) was the first production model of the B-17, and was essentially a B-17A with a larger rudder, larger flaps, and a redesigned nose and 1,200 hp (895 kW) R-1820-51 engines. The gun turret in the upper nose was replaced with a flexible .30 in (7.62 mm) machine gun at its extreme tip, and the bomb-aiming window was replaced with an optical flat in the lower part of the nose fairing.
In October 1942 all planes of the B-17B designation were redesignated RB-17B, R- indicating 'restricted'. The RB-17B was used for training, transport, messenger, and liaison duties. It was in effect a designation of obsolescence.
The "B" series made its maiden flight on June 27, 1939. 39 were built in a single production run, but Army Air Corps serial numbers were scattered over several batches. This was because of limited funding: the Army Air Corps could only buy a few B-17Bs at a time.