The U.S. Air Force had planned for a total of 134 B-50As, RB-50Bs, and B-50Ds to 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 using 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 refueling drums were installed in the rear fuselage and in pods underneath each wingtip. To accommodate these additions, the aircraft's tail had to be lengthened by six feet and a refueling operator's control station was added on each side of the fuselage aft of the pressure shell, with observation blisters. 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.
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 refueling of jet fighters because they were too slow. Conversiona were later made to existing KB-50s to boost the performance. One J47 turbojet engine was added underneath each wing in place of the auxiliary fuel tanks. These conversions were designated KB-50J and KB-50K.
Flight testing of the KB-50J began in April of 1957. The aircraft was able to make successful hookups and fuel transfers at higher altitudes, at greater gross weights, and at higher airspeeds than was possible with the KB-50. The jet engines increased the maximum speed to 444 mph at 17,000 feet at a gross weight of 179,500 pounds. In addition, the jet engines shortened the takeoff distance by 30 percent, and improved the time to climb to refueling altitude by 60 percent. The KB-50J could maintain satisfactory refueling speeds in level flight at altitudes which did not unduly penalize the receiving aircraft.
The Hayes Aircraft Corporation converted 112 KB-50s to KB-50J configuration. The first such aircraft was delivered to TAC on January 16, 1958.
The KB-50s began to be phased out in 1964. Their pumping equipment and jet pods were transferred to KC-97L tankers serving with the Air National Guard. A few KB-50s were still around for the early stages of the Vietnam war, and were pressed into service to refuel jet fighters that were running low on fuel while still over enemy territory. Some of these refuelings were carried out at such low altitudes that they came under fire from the ground. The last KB-50s were retired in 1965.
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