What is this old US Air Force plane?
Based on the unique 6x pusher-prop configuration, this looks like a derivative of the Convair B-36 Peacemaker. Looking up the Wikipedia article for the B-36 and scrolling down to the variants section, this seems like an XC-99 (or, more aptly, the XC-99):



Public domain image via United States Air Force - USAF photo via, Joaobsen, Wagner, Greer (1980), B-36 Peacemaker in action - Aircraft No. 42, Squadron/Signal Publications; 1st edition, ISBN-10: 0897471016

This was to be a passenger variant of the airframe, with a passenger capacity of up to 400 people. Beyond the prototype, it didn't enter service.