From the inception of the B-47 program, it had been recognized that the development of efficient in-flight refuelling techniques would be absolutely essential for the full success of the relatively short-ranged Stratojet bomber. The first USAF in-flight refuelling tankers were converted B-29 and B-50 bombers, but by the mid-1950s the standard USAF tanker was the piston-engined Boeing KC-97. However, the use of the propeller-driven KC-97 as the tanker for the jet-powered B-47 had its drawbacks, since it could not climb to the B-47's best altitude, forcing the Stratojet to descend down to the tanker's level, wasting time and fuel. In addition, the KC-97 tanker was so slow that the B-47 tended to stall during the refuelling operation. In order to keep the B-47 from stalling, the slower KC-97 would often enter a shallow dive to pick up speed--a hair-raising operation for all concerned while the two planes were linked.
Clearly, aerial tankers with greater speeds would be required. In early 1953, two B-47Bs were allocated for tests with the British-developed probe-and-drogue aerial refuelling system. One was to be a tanker and the other a receiver. The converted aircraft were redesignated KB-47G and YB-47F respectively.
The YB-47F was a conversion of B-47B serial number 50-069. It was fitted with a probe in the nose for inflight refueling via the probe-and-drogue system. Unfortunately, the probe-and-drogue refuelling method did not prove to be effective for the B-47, and subsequent models were refuelled by the established flying-boom system. The speed problem was eventually solved by adding auxiliary jet engines to the piston-engined KB-50 and KC-97 tankers, which were used to provided additional bursts of speed during the refuelling operations.