The Titan II had debuted in 1962 as the Air Force's second-generation ICBM to replace the Atlas. By using hypergolic fuels, it could be stored longer and be easily readied for launch in addition to being a simpler design with fewer components, the only caveat being that the propellant mix (nitrogen tetroxide and hydrazine) was extremely toxic compared to the Atlas's liquid oxygen/RP-1. However, the Titan had considerable difficulty being man-rated due to early problems with pogo oscillation. The launch vehicle used a radio guidance system that was unique to launches from Cape Kennedy.

The Gemini-Titan II launch vehicle was adapted by NASA from the U.S. Air Force Titan II ICBM. (Similarly, the Mercury-Atlas launch vehicle had been adapted from the USAF Atlas missile.) The Gemini-Titan II rockets were assigned Air Force serial numbers, which were painted in four places on each Titan II (on opposite sides on each of the first and second stages). USAF crews maintained Launch Complex 19 and prepared and launched all of the Gemini-Titan II launch vehicles. Data and experience operating the Titans was of value to both the U.S. Air Force and NASA.

The USAF serial numbers assigned to the Gemini-Titan launch vehicles are given in the tables above. Fifteen Titan IIs were ordered in 1962 so the serial is "62-12XXX", but only "12XXX" is painted on the Titan II. The order for the last three of the 15 launch vehicles was canceled on July 30, 1964, and they were never built. Serial numbers were, however, assigned to them prospectively: 12568 - GLV-13; 12569 - GLV-14; and 12570 - GLV-15.