The KC-135E became the first all-jet tanker assigned to the Air National Guard in April 1975, and it flew the last operational mission of any KC-135E on 14 July 2009. In the early 1950s, the Strategic Air Command needed a refueling aircraft that could air refuel the B-52 Stratofortress, because the piston-engined KC-97 Stratofreighter then in use could not keep up with B-52s in flight. Boeing therefore designed the jet-powered KC-135 and further refined the flying boom it had developed in the late 1940s to replace the slow hose system. Originally equipped with four Pratt & Whitney J57 turbojet engines, the KC-135 was modified with TF33 turbofan engines in the 1980s, and this version was designated the KC-135E. KC-135-Rs, the only KC-135 flown today, use CFM56 high-bypass turbofan engines that are quieter, cleaner, and more fuel efficient, and the KC-135 fleet, supplemented by the larger McDonnell Douglas KC-10 Extender, is scheduled to be replaced by the Boeing KC-46 Pegasus.