Boeing YB-40 Flying Fortress

The Boeing YB-40 Flying Fortress was a modification of the B-17 Flying Fortress for operational testing, converted to act as a heavily armed gunship supporting 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 quantity production and could not yet escort bombers all the way from England to Germany and back. Work on the prototype, Project V-139, began in September 1942 by converting the second production B-17F-1-BO, with conversion work done by Lockheed's Vega company. It added a second manned dorsal turret in the former radio compartment, twin waist guns, a remotely operated Bendix chin turret, restored cheek guns for 16 guns in all, a bomb bay turned into an ammunition magazine and extra armor. Its gross weight was about 4,000 pounds more than a fully armed B-17.