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.
Here is what must have been a rare event; a 747 tanker refueling a SR-71 Blackbird. I don’t know much information about 747 tankers. If anybody has information about 747 tankers I would love to hear from you. I would especially be interested in photographs of the boom operator’s compartment, controls, and panel.