1972

Strategic Air Command, in request to Air Force headquarters, stated need for a new, accurate missile with a method for deployment capable of surviving a nuclear attack.

1974

Congress approved $37 million for research and development of what was described as "an entirely new intercontinental ballistic missile ICBM ."

1975

Pentagon first used term MX, or "missile experimental," in request for $41 million for research and development. Congress rejected the proposal, questioning survivability of the basing method.

1976

Congress approved $84 million for research and development of advanced ICBM technology, including MX.

Jan., 1977

Outgoing Ford administration, concerned about increasing vulnerability of land-based Minuteman ICBMs, proposed spending program of $294 million to speed up development of MX, described as a potent missile that could be moved from place to place, making it harder to target and hit.

Feb., 1977

President Carter, after entering office, cut budget for MX to $135 million, delaying the project. He cited technical problems with making MX mobile and hopes of arms control agreement with Soviet Union. Congress approved the reduced spending request.

June, 1979

Carter, reversing his earlier stand, announced plans to build the MX and make it mobile to decrease its vulnerability to Soviet attack. Plans called for deploying about 200 missiles by 1986, possibly on railroad cars.

Sept., 1979

Carter approved plan to base new MX missiles in clusters of underground shelters in Utah and Nevada that would have been connected by surface roadways, or "race tracks," allowing the missiles to be shuttled quickly from one shelter to another. Costs of the project were estimated at $33 billion over a 10-year period.

1981 -- present

President Reagan, disagreeing with "race-track" concept, opted for a plan called "dense pack," which grouped the missiles close together and relied on incoming missiles committing "fratricide," a plan rejected by Congress. Reagan then proposed placing 100 MX missiles in existing, hardened silos. Congress so far has approved Reagan's plan for 21 missiles. Last year, Congress approved $1.5 billion for another 21 missiles but said that money could not be released until after March 1. At that point, if the president requests the funds, two votes of approval by both houses are required before the money can be spent.