Actually, there are three 'Air Force One' jets - The ones we'd commonly think of as being 'Air Force 1' (28000 and 29000) which are identical to each other - and a third 747, which is a commercial airliner which will be chartered out/seconded for the day - which might be used if there is a threat to the two other AF1 jets and the pres. has to fly away inconspicuously - or in the event they both break down, aparently! (Yeah, like that'll happen!)
But AF1 isn't a plane, it's a call sign. If President Bush decided to - and this is more likely to happen than both the normal 742's breaking down - take the wright flyer, a DC3 or a Handley Paige it would be classed as Air Force 1.
The only reason those jets are used is because you can control the world from them, as you would from the whitehouse. (If the world chooses to listen
) And they have misiles, and a lot of security stuff you aren't supposed to know about, which is why it's used all the time, especially under Double-ya.