Boeing B-17 Flying Fortress YB-40

Before the long-range P-51 Mustang was available as an escort, the YB-40 was trialled as a bomber converted into an escort "gunship": it added a second dorsal turret above the radio operator's position, doubled the guns at each waist position, and replaced the bombing equipment with a remotely-operated chin turret carrying two .50-cal guns, with the bomb bay converted into an ammunition magazine. The type was withdrawn because, once standard bombers dropped their bomb loads, the much heavier YB-40 could no longer keep pace and fell behind; it was pulled after just fourteen missions. 26 were built in total: one XB-40 prototype, 21 pre-production YB-40s and 4 TB-40 trainers. The chin turret it introduced later became standard on late B-17Fs and the B-17G.