This is a bit of a borderline case between a real and never built aircraft:
The Boeing 707-700, or 707-720.
In 1979 Boeing converted one of its last 707-320C airframes to operate as a testbed for the CFM56 engines to study the fleasibility of retrofitting older 707s with the new powerplants (like douglas will do wit its super-70s program later). Fearing that the re-engined aircraft could threaten the 757 sales market boeing abandoned the plan and converted back the airframe to -320C specification before selling it to Moroccan air force.
Studies made on the 707-700 prototype were later implemented for military purposes, like re-engined USAF CK-135 and late-builD E-3 Sentry and E-6 Mercurys.