Prior to the introduction of the P-51 Mustang, a B-17 escort variant called the YB-40 was introduced. This aircraft differed from the standard B-17 in that a second dorsal turret was installed between the top turret and the waist guns; and the single 0.50-calibre (12.7 mm) machine gun at each waist station was replaced by a pair of 0.5-calibre (12.7 mm) guns.
In addition, the bombardier’s equipment was replaced with two 0.50-calibre (12.7 mm) machine guns in a ‘chin’ turret to augment the existing ‘cheek’ machine guns, and the bomb bay itself was converted to a magazine.
The concept was twofold. First, the YB-40 would provide a heavily-gunned escort capable of accompanying the bombers all the way to the target and back.
Second, they were used as decoys; a YB-40 would leave the bomber stream with one engine feathered, apparently in distress. Enemy fighters would close for the kill and discover that the 'cripple' was nothing of the kind.
The aircraft was used with some success in the Mediterranean, but was overall a failure because it could not keep up with standard B-17Fs once they had dropped bombs. It was withdrawn from service after less than ten missions. (×25: 1 XB-40, 20 YB-40, 4 TB-40)