Boeing proposed a 747 adapted as an aerial refueling tanker during the 1970s Advanced Cargo Transport Aircraft (ACTA) program that produced the McDonnell Douglas KC-10A Extender for the USAF. This is a summary of the aircraft modifications and flight tests of the proposed KC-33.
The Boom Operator’s station in the KB-29P was in approximately the same location as where the tail gunner would sit in a conventional B-29. A large hemispherical Plexiglas bubble provided a generous view of the refueling operation. The boom was mounted beneath the operator which made the job of effecting a contact more difficult. Subsequent aircraft would be configured with the boom mounted above the operator so that he could more accurately sight down the boom.
Boeing developed the rigid flying boom system to improve on the hose and drogue in-flight refueling (IFR) system. The boom, mounted at the aft-most portion of the KB-29P, was fitted with two small wings that allowed the boom operator to maneuver the boom. The pilot of the receiver aircraft, guided by the boom operator and light signals on the tanker belly, flew behind and below the tanker for refueling. Once in position, the boom operator “flew” the boom into the refueling receptacle, and the KB-29P flight engineer began fuel transfer.
The flying boom system became the most common method for IFR and was used on KB-50s and KC-97s. It is still used on the USAF’s modern tankers — the KC-135 and KC-10.