At the time of its development, long-range fighter aircraft such as the North American P-51 Mustang were just entering quantity production, and thus were not yet available to accompany 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 (serial number 41-24341) built.

[2] The YB-40's mission was to provide a heavily armed escort capable of accompanying bombers all the way to the target and back.

Of the initial order of 13, one (serial 43-5732) was lost on the delivery flight from Iceland to the UK in May 1943; it force-landed in a peat bog on a Scottish island after running out of fuel.

Five confirmed and two probable German fighter kills were claimed, and one YB-40 was lost, shot down on 22 June mission to Hüls, Germany.

Luftwaffe fighter chief Adolf Galland considered the gunship's handful of combat victories to be "insignificant" and not worth the cost of the aircraft.

[3] The increased weight from the additional machine guns and ammunition nearly cut the YB-40's climb rate in half from that of a B-17F, and in level flight it had difficulty keeping up with standard Flying Fortresses, especially after they had dropped their bombs.

42-5736 ("Tampa Tornado") was flown to RAF Kimbolton on 2 October 1943 where it was put on display and later used as a group transport.