The first RC-135 variant, the RC-135A, was ordered in 1962 by the United States Air Force to replace the Boeing RB-50 Superfortress. Originally nine were ordered but this was later reduced to four. Boeing allocated the variant the designation "Boeing 739-700" but they were a modified variant of the KC-135A then in production. They used the same Pratt & Whitney J57 turbojet engines as the tanker, but carried cameras in a bay just aft of the nose wheel well where the forward fuel tank was normally located. They had no in-flight refueling system and they were to be used for photographic and surveying tasks. Although the RC-135A was the first designation in the RC-135 family, it was not the first RC-135 in service. That distinction belongs to the RC-135S, which began operational reconnaissance missions in 1961, followed by the RC-135D in 1962.