The Air Force wants to base the first 40 MX missiles in Minuteman III silos in North Dakota or Wyoming, according to informed sources.

The Minuteman III silos are favored, sources said, because:

* They have the most modern hardened command and control structure.

* Each Minuteman III missile field contains 150 or more intercontinental ballistic missiles, so choosing a Minuteman III field would allow expansion to more than 40 MX missiles.

* The MX field would be small enough in area to permit the building of an antiballistic-missile defense system, if that is the direction the Reagan administration decides to take in 1984.

The Air Force plan has some drawbacks, according to a congressional military specialist who has reviewed it. If the administration decides to harden the Minuteman III silos even further, he said yesterday, it would have to enlarge their size and thus violate the provisions of the SALT II treaty that limit the modification of existing land-based silos.

Although the MX missile is larger than the Minuteman III, it would fit into the Minuteman silo, the specialist said, but there would not be enough extra space for additional concrete hardening.

The Air Force originally favored the Carter administration's Nevada-Utah multiple-basing scheme for 200 MX missiles. But President Reagan canceled the Carter program, and Defense Secretary Caspar W. Weinberger ordered the Air Force to come up with the best ways for placing the giant new ICBM in silos.

The Reagan administration program, announced Oct. 2, called for the production of 100 operational MX missiles, but provided for temporary silo basing for 40 or fewer between now and 1984.

The administration said that by that date it would decide whether additional missiles would go into silos or in two other deployments being studied: in underground silos or aboard continuously patrolling aircraft.

Earlier this month, in an amendment to the fiscal 1982 defense money bill, Congress directed that the Pentagon also study a deceptive basing concept.

When the president's MX decision was first announced Weinberger said that he wanted to put 36 MX missiles in super-hardened Titan II missile silos. The large Titan silos seemed to be the perfect place for the MX, because the 18-year-old Titans were going to be retired beginning in fiscal 1983.

But when it was pointed out that it would cost much more to modify Titan silos and that 36 MXs would have to be spread over two states because there are only 18 Titan silos in any one state, Weinberger modified his directive to the Air Force, saying the service should also look at using Minuteman silos or even a combination of the two.

The Air Force sent its proposals to Weinberger more than a month ago and, sources said, is awaiting orders on which plan to implement.

One Air Force officer said he believed Weinberger was waiting for the president to sign the fiscal 1982 defense appropriations bill before moving ahead with plans that would use fiscal 1982 money. That bill was signed yesterday.

By placing 36 MXs in Titan II silos the Air Force could avoid keeping the program within the constraints of SALT II. The cost, however, would be $7.8 billion, according to a Pentagon fact sheet distributed earlier this month on Capitol Hill. That same document points out that it would cost $2.2 billion less to deploy 40 MX missiles in Minuteman III silos.

Other alternative plans include putting the MXs in older, Minuteman II silos in Montana, South Dakota or Missouri. The problem with these silos, sources said, is that they have less sophisticated command and control facilities and less hardening than the Minuteman III silos.

If Weinberger approves the Air Force Minuteman III program, sources said, the first missiles could be installed late in 1985 or early 1986 and the entire program completed during fiscal 1987.