The Air Force has retired the last of the KC-135E Stratotanker aerial refueling tankers that saw service since the 1950s. The planes, built on the airframe of the Boeing 707 commercial airliner, will be replaced by the planned KC-X. The final active-force KC-135E landed at Sheppard Air Force Base, Texas, on Oct. 5. Until the KC-X joins the inventory, KC-135R and KC-10 Extender aircraft will fulfill the aerial refueling mission.