Both B-17 struting Their Ball Turrets

The B-17E was the first version of the Fortress to be produced in large
numbers. It was originally designed to correct some of the deficiencies in the earlier Fortresses that had been brought to light as a result of combat reports coming in from Europe. The modifications which resulted in the B-17E were destined to turn an airplane which had been a relative failure into an outstanding success.
It is often written that the B-17E was the result of initial combat experience with the B-17C and D during the first months of combat in 1942 in the Pacific against the Japanese. Other sources report that the B-17E had its origin in the negative experience that the RAF had with the Fortress I (B-17C) over Europe in the summer of 1941. Neither view is correct, since the B-17E was first ordered on August 30, 1940, and the first prototype took to the air on its maiden flight on September 5, 1941.
The crews of early Fortresses (primarily B-17C and D) were well aware of the existence of a rather severe blind spot immediately to the rear, making it almost impossible to direct defensive firepower against an enemy fighter approaching from that direction. Early USAAC Fortress pilots had been forced to devise some rather ingenious maneuvering techniques to get around this problem--when attacked from the rear they would jink the aircraft back and forth, making it possible for the left and then the right waist gunners alternatively to get in fleeting shots at the attacker. However, in the tight formations that became de rigeur during missions over Germany during 1943-44, such maneuvering would have been downright dangerous.
In search of a better solution, the B-17E introduced a completely new rear fuselage with a manually-operated turret housing two 0.50-inch machine guns fitted in the extreme tail. In order to accommodate the tail gun, the fuselage of the B-17E was a full six feet longer than that of the D.
A Bendix electrically-powered turret containing two 0.50-inch machine guns was installed on the upper fuselage immediately behind the flight deck. This turret was usually operated by the flight engineer. The oval waist positions were replaced by rectangular apertures with removable windows. A single 0.50-inch machine gun could be mounted behind each of these windows.
A power-operated belly turret replaced the ventral "bathtub" housing of the B-17D. This turret was remotely-controlled by a system of mirror periscopic sights from a Plexiglas bubble below the waist hatches.
In order to achieve better stability during the bomb run, the span of the horizontal tailplane was increased, the vertical tail was greatly increased in area, and a long dorsal fin was fitted in front of the tail.
The first B-17E flew on September 5, 1941. It immediately superseded the B-17D on the production line, and as fast as they could leave the production line, they were issued to operational units.