06/03/2008. From Joe Baugher's website: "As the Model V-139, the XB-40 prototype was produced in November of 1942 by the Vega Division of Lockheed. They converted a standard Boeing-built B-17F (s/n 41-24341) to escort configuration by adding a dorsal turret in the radio compartment position carrying a pair of 0.5 in (12,7 mm) machine guns, a chin turret underneath the nose equipped with a pair of 0.5 in machine guns and twin gun mounts instead of the usual single gun mounts at each waist position.

XB-40 (41-24341) (Hartley Goodale Collection)

The regular top, belly and tail turrets were retained, bringing the total defensive firepower to fourteen 0.5 in machine guns. Additional protective armour was fitted for better crew protection. The bomb bays were replaced by storage areas which carried additional ammunition for the guns. The normal ammunition load for the aircraft was 11,135 rounds, which could be increased to 17,265 rounds with a reduced fuel load.

XB-40 (41-24341) (Alfred Damen Collection)

Twenty Vega-built B-17Fs were converted to YB-40 configuration, plus four TB-40 trainers and the first operational YB-40 sortie took place on May 29, 1943 against St. Nazaire. Eight other missions were later flown, the last one taking place on July 4, 1943.

Very early on, it was found that the net effect of the additional drag of the turrets and the extra weight of the guns, armor, and additional ammunition was to reduce the speed of the YB-40 to a point where it could not maintain formation with the standard B-17s on the way home from the target once they had released their bombs. The YB-40 could protect itself fairly well, but not the bombers it was supposed to defend.

Consequently, it was recognized that the B-40 project was an operational failure, and the surviving YB-40s were converted back to standard B-17F configuration or used as gunnery trainers back in the USA."