In 1956, the Air Force endorsed a program to improve the performance of the KB-50 aerial tanker fleet. The piston-engined KB-50 made good tankers for the other B-50s in SAC's arsenal, but they lacked both the speed and altitude performance to handle the new generation of supersonic fighter bombers that were just then beginning to come into service. This performance enhancement was obtained by adding a 5200 lb.s.t. General Electric J47-GE-23 turbojet housed in a pod suspended from a pylon at the former location of each of the KB-50's auxiliary underwing wing tanks. The designation KB-50J was assigned to the conversion.
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 refuelling altitude by 60 percent. The KB-50J could maintain satisfactory refuelling 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-50J was considered only an interim tanker, pending the availability of the KC-135. Unfortunately, the elderly KB-50Js began to deteriorate almost as soon as they were delivered, and TAC was forced to resort to cannibalization to keep at least some of the retrofitted tanker aircraft flying. The inner liner of KB-50 fuel cells began to crack, forcing Hayes to exchange DB-50 fuel cells for new similar ones or for fuel cells that had been taken from B-50s that had been consigned to storage at Davis Monthan AFB in Arizona. Landing gear malfunctions were frequent, and many parts started to break simply because of old age. More and more maintenance had to be performed in order to keep the aircraft flying.
The KB-50Js were all converted B-50Ds and were all orginally designated KB-50. The KB-50Js 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 refuellings were carried out at such low altitudes that they came under enemy fire from the ground. The last KB-50s were retired in 1965.
47-163/170 Boeing B-50D-65-BO Superfortress c/n 15847/15854 170 later converted to KB-50D, prototype for later KB-50J and KB-50K programs. 48-046/052 Boeing B-50D-70-BO Superfortress c/n 15855/15861 046 converted to KB-50D, prototype for subsequent KB-50J and KB-50K programs. 0049 to KB-50J 48-053/062 Boeing B-50D-75-BO Superfortress c/n 15862/15871 0059 to KB-50J 48-063/072 Boeing B-50D-80-BO Superfortress c/n 15872/15881 0066 to KB-50J 48-0113/0127 Boeing B-50D-105-BO Superfortress c/n 15994/16008 0123 to KB-50J 49-0293/0325 Boeing B-50D-115-BO Superfortress c/n 16069/16101 0321 to KB-50J 49-0326/0358 Boeing B-50D-120-BO Superfortress c/n 16102/16134 0344,0350,0356 to KB-50J 49-0359/0391 Boeing B-50D-125-BO Superfortress c/n 16135/16167 0373,0378,0389,0391 to KB-50J 49-0389 on display at WPAFB museum, marked as KB-50J 48-0014