B-17B
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 small gun turret in the upper nose blister was replaced with a .30 in (7.62 mm) machine gun, its barrel run through a flexible ball socket; the separate bombadier's window was replaced with a flat plex-i-glass panel in the lower center of the nose combing. During Army Air Force service, its bulged machine gun blisters were replaced with the more aerodynamic flush side windows used on the B-17C/D
In the interior of the plane, crew members were rearranged. Many internal systems were revised. Most notable of these was a switch from pneumatic to hydraulic brakes.
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.
Read more about this topic: Boeing B-17 Flying Fortress Variants