Page "Heavy bomber" Paragraph 6

The American innovation of the manned, ventrally-mounted Sperry ball turret on the B-17 and B-24 bombers was a virtually self-contained defensive weapon system that rotated a full 360 degrees horizontally with a 90-degree elevation, and its pair of M2 Browning machine guns had an effective range of one thousand yards.

The B-29 Superfortress, introduced later in the war brought the innovation of four remotely operated twin-gun turrets on its fuselage, controlled through an analog computer sighting system sighted from any of a trio of hemispherical clear-glazed stations, with only the rear tail gunner position on the plane directly manned by a crewmember.