The substantial design changes between the B-57A and B-57B models resulted in delays to the overall program, as well as incurring substantial costs on the fixed-price contract. Gunston and Gilchrist stated of the consequences to the program: "This was clearly not the manufacturer's fault, but the money effectively ran out after 177 of the planned 250-aircraft order had been completed". In 1954, in response to the sound performance offered by the B-57B, the USAF elected to place a large second order; this included a further 100 B-57B bombers, 38 "B-57C" trainers, and 20 "RB-57D" high-altitude reconnaissance aircraft. In 1955, a final batch of 68 "B-57E" target tugs was ordered.