The Air Force had planned that a total of 134 B-50As, RB-50Bs, and B-50Ds would be converted to aerial tankers, once they were no longer needed by the atomic bombing forces of SAC. All of their armament would be deleted, the outer wings would be reinforced, and equipment would be added that would make it possible for them each to refuel three fighter aircraft simultaneously by the probe-and-drogue method.
The designation KB-50 was assigned to these conversions. The contract for the modification of the B-50s was assigned to the Hayes Aircraft Corporation, Additional fuel tanks were installed in the bomb bay and A-12B-1 refuelling drums were installed in the rear fuselage and in pods underneath each wingtip. The tail was lengthened by six feet. One hose was unreeled from the lengthened tail, and one hose was unreeled from a pod that was carried underneath each wing tip. The original B-50-type 700-gallon auxiliary wing tanks were retained. A refuelling operator's control stations was added on each side of the fuselage aft of the pressure shell, with observation blisters.
Two B-50Ds (47-170 and 48-046) were modified to serve as prototypes for subsequent tanker conversions. They were designated KB-50D. The completion date for the Hayes modifications was tentatively set for December of 1957, but the project proceeded so smoothly that it was actually completed ahead of time. The KB-50D had a lengthened tail cone which housed a drogue-type refuelling hose. Additional refuelling hoses were fitted in pods mounted underneath the wing tips.
The first KB-50 entered service with the Tactical Air Command in January of 1956. They replaced the KB-29s previously operated by TAC, which had proven to be totally unsuitable for the refuelling of jet fighters because they were too slow. By November of 1957, TACs KB-29s had all been phased out, and by the end of 1957, all of TACs aerial refuelling squadrons had received their full complement of KB-50s. TAC was quite pleased with its new tankers, since they presented no serious problems and were quite reliable.
No distinction was made at first between different series of B-50 aircraft that had been modified, and the tankers were all identified only as KB-50. However, the structural and equipment differences between different series made separation necessary for maintenance and operations, and distinguishing designations were eventually applied.
The increasing performance of operational TAC fighters made it necessary to boost the performance of the tankers, and this was done by adding a J47 turbojet engine underneath each wing in place of the auxiliary fuel tanks. These conversions were designated KB-50J and KB-50K.