What is this white-tail, short upper deck 747 I just saw? It was seen taking off from a US Air Force base, which is part of Air Mobility Command.  A great deal of their operation is departing or returning troops from abroad, so it gets a lot of rare visits from oddball commercial aircraft.  

This 747 had the short 3-window upper deck area, so it was a -100,-200 or SP, and it wasn't an SP.    It was a "white tail", entirely white except for some numeric markings.  

No USAF marking, that's not a surprise since the Air Force has six 747-100/200s: all accounted for by four NEACP planes (why would they come to an AMC base?) and two VIP transports (and they have a distinctive paint scheme, so, I wouldn't be asking). 

It did not carry the distinctive markings of several Arab state head-of-state aircraft, the NASA paint scheme, nor the Las Vegas Sands Corp. scheme, nor Earnest Angley Ministries.   It did not have any funny engines on it.  Kalitta Air doesn't fly them anymore.  

I have to assume none of the nine in Iranian service would be visiting a USAF/AMC base. 

I'm sorry, I tried to get a photo, but could not safely stop.  What could this airplane be?
All factory freighter 747s have short upper-decks, that includes the -400F and -8F. Some don't, but those are converted from passenger 747s that had the stretched upper-deck.

It could have been a 747-400F contracted by the military, plenty of those come in all-white, such as N404KZ:


source

I don't know on what basis the military prefers contractors over C-5s, etc., but an example of a civilian 747 contracted by the Air Mobility Command that crashed is National Airlines Flight 102.