the titan rocket family was established in october 1955, when the air force awarded the martin company a contract to build an intercontinental ballistic missile (icbm). it became known as the titan i, the nation's first two-stage icbm and first underground silo-based icbm. more than 140 titan ii icbms, once the vanguard of america's strategic deterrent force, were built. titan iis also were flown in nasa's gemini manned space program in the mid-1960s. the titan 23b is a derivative of the titan ii vehicle with an agena d upper stage added.