What was the YB-40?

The Boeing YB-40 Flying Fortress was a modification of the B-17 Flying Fortress bomber, built for operational testing purposes, converted into a heavily armed gunship to support other bombers during World War II. When it was developed, long-range fighters such as the North American P-51 Mustang were only just entering full-scale production and were not yet available to escort bombers from England to Germany and back. It differed from the standard B-17 in having a second manned dorsal turret, installed in the former radio compartment, just behind the bomb bay and ahead of the ventral ball turret position. The bombardier's equipment was also replaced by two Browning AN/M2.50-caliber light-barrel machine guns, housed in a remotely operated Bendix-designed chin turret.

The aircraft's gross weight was about 4,000 lb (1,800 kg) greater than that of a fully armed B-17. The added weight of the guns and ammunition nearly halved the YB-40's climb rate compared to the B-17F. While the B-17F it was based on was rated to reach 20,000 ft (6,100 m) in 25 minutes, the YB-40 was listed at 48 minutes. The XB-40's first flight took place on 10 November 1942.

Seven YB-40s were dispatched on the raid over Saint-Nazaire, but they could not keep pace with the B-17s on the return from the target. Adolf Galland, head of the Luftwaffe's fighter arm, considered the gunship's handful of combat victories "insignificant" and not worth the aircraft's cost. Despite the project's overall failure as an operational aircraft, it led directly to the Bendix chin turret being fitted on the last 65 aircraft built by Douglas — 86 according to some sources — starting with the B-17F-70-DL production block.