The original B-17 type, the Y1B-17, first flown in July 1935, was used, with some modifications, until 1939. This plane revolutionized heavy bomber design.
In the B-17B, delivered in 1939, the gun turret was removed from the nose.
Flush windows in place of blister turrets distinguished the B-17C and B-17D, first produced in 1940 and 1941 respectively.
The completely redesigned B-17E appeared just before Pearl Harbor and provided a tail-gun post in a longer fuselage, with a Sperry ball turret under the fuselage, and another power turret on top.
A plexiglas nose and greater bomb capacity were features of the B-17F, which appeared in the spring of 1942.
The B-17G arrived in 1943, adding a chin turret operated by remote control and glass-enclosed gun positions on the sides of the fuselage.