Air Force One is not a single aircraft, but any Air Force aircraft carrying the president.  In 1958, the Air Force commissioned two Boeing 747-2G4B (modified, of course) in 1972.  One jet, with a tail number of SAM 27000 (Special air mission) was specially made for Richard Nixon.  There was also another jet with a tail number of 26000.

            The 27000 continued to perform most of the missions, and the 26000 was only a backup which was rarely used.  Each flight is categorized as a military operation.  The 27000 carried President Ronald Reagan to Berlin when he made his famous speech urging the Soviet government to tear down the Berlin Wall.

            In 1990, two new Boeing 707-320B jets were commissioned with tail numbers of 28000 and 29000.  The 26000 was decommissioned, but the 27000 continued to carry the President until only a few years ago.

            The interior of the two modern-day aircraft was completely remodeled specially for the President.  This includes office areas, two kitchens, a medical operating table, pharmacy, sleeping quarters, communications, phones, TVs, and even a workout area.  There is more space for the first family and the media.  Its arsenal include air to air missiles in case of an attack, and the plane can be operated as the military command center in case Washington is destroyed.  Air Force One has also been modified to include in-flight refueling.

            New callsigns, were established during Dwight D. Eisenhower's presidency, partly because another aircraft with the same callsign entered the same airspace, and there was much confusion.  Air Force One is the designation for any Air Force aircraft with the president on board.  An Army airplane carrying the president would be called Army One.  The President frequently flies within Washington on a Marine Corps helicopter designated Marine One.  Similarly, any Navy aircraft carrying the president is designated Navy One.  Any civilian plane carrying the president, which rarely happens, is designated Executive One.  Any plane carrying anyone in the first family is called Executive One Foxtrot.

            Air Force One has come to symbolize the power of the president, and will continue to do so for many years.

Sources:

            http://en.wikipedia.org/wiki/Air_Force_One.html