 Because General Dynamics was responsible for contract maintenance on the D model, its Fort Worth Division was given the sole-source contract for the development of the "Pee Wee III" RB-57F prototypes.The two aircraft chosen for initial development were Martin B-57Bs 52-1559 and 53-3864, which supplied the fuselage and horizontal stabilizers around which the rebuild was made.The prototype RB-57Fs incorporated many major changes from the RB-57D, the most obvious of which was an enlarged computer-designed wing to enable it to operate at extreme altitudes.