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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. :ooh: 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 It was withdrawn from service after less than ten missions.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)

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