Air Force One is the official air traffic control-designated call sign for a United States Air Force aircraft carrying the president of the United States. The term is commonly used to denote U.S. Air Force aircraft modified and used to transport the president, and as a metonym for the primary presidential aircraft, VC-25, although it can be used to refer to any Air Force aircraft the president travels on.
The idea of designating specific military aircraft to transport the president arose during World War II when military advisors in the Department of War were concerned about the risk of using commercial airlines for presidential travel. In 1944, the Douglas C-54 Skymaster was converted for use as the first purpose-built presidential aircraft. Dubbed the Sacred Cow and operated by the U.S. Army Air Force, it carried President Franklin D. Roosevelt to the Yalta Conference in February 1945 and was used for another two years by President Harry S. Truman.