What was Boeing's competitor to the C-5?
In 1964 the US Air Force awarded study contracts for a new cargo aircraft to Boeing, Douglas and Lockheed. The aircraft was called CX-HLS. ( cargo experimental - heavy logistics system)

Lockheed won with the C-5A design. Boeing's design was never built but would have looked like this:



source: Widebody Aircraft Chronology



Source: Boeing archives



Source:TDPRI.COM