The Boeing KB-29M, briefly designated KB-29K, was a modification of a production B-29 or B-29A for in-flight refueling (IFR). The B-29MR, briefly designated B-29L, was also a modification of a production B-29 or B-29A used as a receiver aircraft for the refueling operation. The first system used required the refueling aircraft to unreel the refueling hose to the receiver aircraft, which would snag the hose and reel it in. The hose was then connected to internal fuel cells and fuel transfer began. The system was quite awkward but saw operational use in the late 1940s. The most famous use of this technique was for the flight of the B-50A Lucky Lady II, the first aircraft to complete a non-stop around the world flight.
Later, some KB-29Ms were fitted with more efficient probe and drogue in-flight refueling systems. The KB-29M unreeled the refueling hose which had a donut-shaped para-drogue attached to the end. The receiving aircraft had a refueling probe mounted on the nose or wingtip and the pilot flew his aircraft's probe into the refueling hose receptacle for fuel transfer. The first combat IFR was accomplished using this technique on May 29, 1952, when 12 F-84Es of the 159th Fighter-Bomber Squadron flew a bombing mission from Itazuke Air Base, Japan, against targets at Sariwon, North Korea.
The probe and drogue in-flight refueling technique is still used today primarily by helicopters.