That was the original mock-up of the Phantom II, called the F3H-G. It was originally designed as a single-seater, then the Navy wanted it changed to a two-seater. The two-seater looked almost identical to the above picture, except for an extra canopy behind the pilot, and the horizontal stabilizers were angled downward just a bit (like the Phantom II we all know and love today). This two-seat version was originally called the AH-1 in 1954, and was then changed to F4H-1 in 1956, to reflect its roll as a fighter instead of an attack aircraft (F=fighter 4=fourth design from H=McDonnell -1=1st production version of that particular aircraft). In 1962, the DoD decided to standardize all military aircraft designations, and chose the USAF way of doing that, so the F4H-1 became the F-4.