The XC-99 was a transport version of the Convair B-36 Peacemaker bomber. It first flew in 1947. Pan Am acually took out options on a proposed civil version but like the Air Force they found it too big and expensive for the passenger traffic of that time.
The sole XC-99 was later based at Kelly AFB, TX and used as a transport. It actually had the lowest operating cost of any USAF transport of its day. It was retired in 1957 and put on display outside the gates of Kelly.
The a/c was a landmark in San Antonio for many years, but time and vandalism took their toll. A few years back the Air Force Museum took custody of the XC-99 and moved it back to Kelly AFB. With Kelly's upcoming closure, the XC-99 is facing an uncertain fate.