Air Force One is the official air traffic control call sign of any United States Air Force aircraft carrying the President of the United States. In common parlance the term refers to those Air Force aircraft whose primary mission is to transport the president; however, any U.S. Air Force aircraft may carry the "Air Force One" call sign while the president is on board. Air Force One is a prominent symbol of the American presidency and its power, with the aircraft being the most famous and most photographed in the world.
The idea of designating specific military aircraft to transport the President arose in 1943, when officials of the United States Army Air Forces – the predecessor to the U.S. Air Force – became concerned with relying on commercial airlines to transport the President. A C-87 Liberator Express was reconfigured for use as a presidential transport; however, it was rejected by the Secret Service amid concerns over the aircraft's safety record. A C-54 Skymaster was then converted for presidential use; this aircraft, dubbed the Sacred Cow, transported President Franklin D. Roosevelt to the Yalta Conference in February 1945, and was subsequently used for another two years by President Harry S. Truman.
The "Air Force One" call sign was created after a 1953 incident involving a flight carrying President Dwight D. Eisenhowerentered the same airspace as a commercial airline flight using the same call sign. Several aircraft have been used as Air Force One since the creation of the presidential fleet. Since 1990, the presidential fleet has consisted of two Boeing VC-25As – specifically configured, highly customized Boeing 747-200B series aircraft. The Air Force is currently looking into replacing the two aircraft used as Air Force One, with Boeing the only contender. The Air Force expects three aircraft, delivered in fiscal 2017, 2019 and 2021.