In addition to greater payload capability, the Titan II promised greater reliability than the Atlas LV-3B which had been selected for Project Mercury, because Titan's hypergolic-fueled engines contained far fewer components.
Several modifications were made to the Titan missile to man-rate it for Project Gemini:
Redundant systems were fitted to reduce the chances of launch failures.
To help guard against the possibility of a guidance malfunction causing the engine nozzles to gimbal hard right or left, an extra backup guidance system was added.
The second stage propellant tanks were lengthened for longer burn time and unnecessary vernier engines and retro-rockets were removed. Because the second stage engine had had issues with combustion instability, it was equipped with baffled injectors.
Modifications were made to the tracking, electrical and hydraulics systems in the interest of improved reliability.
Modifications were overseen by the Air Force Systems Command. Aerojet, the manufacturer of the Titan's engines, had released a revised model during mid-1963 due to deficiencies in the original design and also attempting to improve manufacturing procedures.
Assembly was done at Martin-Marietta's Baltimore plant so as not to interfere with missile work at the Denver facility, although it also saved the former from a planned shutdown.