This was the second Guppy built and was called the "Super Gupy". Built from a Pan American 377 Stratocruiser with wings from an Air Force YC-97J with turbo-prop engines.
NB-52A Stratofortress "The High And Mighty One", this aircraft, one of 3, was used as a test vehicle and carried the X-15 aloft. Each X-15 symbol represents an X-15 flight that was launched from this aircraft.