1. Any plane can be ‘Air Force One’
“Air Force One” is not the name of one particular plane. Its the name given to any plane that the President of United States is travelling at that particular moment. Basically a radio call name given to the plane carrying the president in order to avoid confusion with other planes travelling in that particular area. Similarly the Army aircraft the president is travelling is called Army One, his helicopter is called Marine One.
However in modern culture, the term ‘Air Force One’ has come to be used for the particular custom made plane.