In 1944, the War Department was aware of aviation advances in Germany and issued a requirement for a range of designs for medium bombers weighing from to more than . Other designs resulting from this competition, sometimes named the "class of '45", included the North American XB-45 and the Martin XB-48. Procurement began with a letter contract (cost-plus-fixed-fee) on 17 January 1945 with mockup inspection and approval in early February. Orders for three prototypes followed on 27 February 1945 with certain changes recommended by the board. Serials "45-59582" to "59584" were assigned. Budgetary concerns also led to the contract being changed to a fixed-price type.