By the time planning began for the final Minuteman deployment area, the Air Force had developed a vastly improved version of the missile. Called Minuteman II, the new missile offered improved range, greater payload, more flexible targeting, and greater accuracy, leading one Air Force spokesperson to estimate that its "kill capacity" was eight times that of Minuteman I. Minuteman II was deployed first at Grand Forks AFB, North Dakota. In September 1965, South Dakota Congressman E. Y. Berry announced that the Ellsworth AFB facilities would also receive the new missile system. According to Berry, Minuteman II would help Ellsworth AFB remain "one of the nation's most important military installations." In October 1971, Boeing began refitting the Ellsworth silos to accommodate Minuteman II, and completed the project in March 1973.
In May 1964, the Soviet Union displayed a battery of anti-ballistic missiles in Moscow's Red Square, prompting concern about the vulnerability of Minuteman I and II missiles. The following year, the Air Force began to develop an even more advanced version of the missile. By late summer of 1968, Minuteman III was ready for testing. Longer and more powerful than its predecessors, Minuteman III offered an improved guidance system that could be retargeted in minutes. But, according to the New York Times, the missile's "most telling advantage" lay in its "revolutionary new warhead: the MIRV, or multiple independently targeted reentry vehicle.'' The MIRV could deliver three hydrogen bombs to widely scattered targets, a capability that would "render current and contemplated antimissile defense systems largely inadequate," and "thrust the world into a new era of weapons for mass destruction."
The Air Force deployed Minuteman III at Warren, Minot, Grand Forks, and Malmstrom Air Force Bases, and extensively modified the Minuteman launchers at these locations to accommodate the new missiles. Each launch tube was equipped with a new suspension system that could hold the missile absolutely motionless during the aftershocks of a nuclear attack. The Air Force also installed a system of seals, filters, and surge arrestors designed to prevent electronic equipment from being damaged by the powerful electromagnetic waves generated during nuclear explosions.
In July 1975, when the last of the Nation's 550 Minuteman III missiles was lowered into its silo at Malmstrom AFB, Montana, only 450 Minutemen II remained in the American arsenal - at Malmstrom, Ellsworth, and Whiteman Air Force Bases. This force structure remained intact for nearly two more decades.