The McDonnell Douglas KC-10 Extender is an aerial refueling tanker aircraft operated by the United States Air Force (USAF). A military version of the three-engined DC-10 airliner, the KC-10 was developed from the Advanced Tanker Cargo Aircraft Program.[N 1] It incorporates military-specific equipment for its primary roles of transport and aerial refueling. It was developed to supplement the KC-135 Stratotanker following experiences in Southeast Asia and the Middle East. The KC-10 was the second McDonnell Douglas transport aircraft to be seleced by the Air Force following the C-9. A total of 60 KC-10s were produced for the USAF. The Royal Netherlands Air Force operates two similar tankers designated KDC-10 that were converted fro DC-10s.