The KC-10 Extender is a McDonnell Douglas DC-10 Series 30CF variant converted to the aerial tanker role. It beat Boeing’s Model 747 aerial tanker variant to win Advanced Tanker Cargo Aircraft competition in 1977, and 60 of them were delivered to the USAF between March 1981 – November 1988. The KC-10s serve as the USAF’s current “heavy tanker” option, with much greater fuel and cargo capacity than the Eisenhower-era KC-135/707s. The KC-10s also have an unusual ability within the US tanker fleet – they can take on fuel from other aerial tankers to fill themselves up, then send the others back to base. The USA’s new KC-X tankers will also have this ability, but the older KC-135s do not, except for a handful of KC-135R/T models.