Design and developmentClose-up of the array of .50-caliber guns on the Boeing YB-40 Flying Fortress.Work on the prototype, Project V-139, began in September 1942 by converting the second production B-17F-1-BO (serial number) built. Conversion work was done by Lockheed's Vega company.The aircraft differed from the standard B-17 in that a second manned dorsal turret was installed in the former radio compartment, just behind the bomb bay and forward of the ventral ball turret's location. The single .50-caliber light-barrel (12.7 mm) Browning machine gun at each waist station was replaced by two of them mounted side by side as a twin-mount emplacement, with a mount for each pair of these being very much like the tail gun setup in general appearance. The bombardier's equipment was also replaced by two .50-caliber light-barrel Browning AN/M2 machine guns in a remotely operated Bendix designed "chin"-location turret, directly beneath the bombardier's location in the extreme nose. (Role: Bomber Escort) (Made: 1942) (Retired 1943)