The Air Force, in the first step toward what may become a $20 billion program to develop and produce a fleet of revolutionary "Stealth" bombers, announced yesterday that it has selected Northrop Corp. as prime contractor for initial research and development work on the project.

"Stealth" is a top-secret and top-priority Pentagon project to develop a new type of bomber virtually invisible to enemy radar and thus able to carry bombs and missiles through heavy air defenses such as those ringing the Soviet Union. The idea is to combine a variety of new aircraft shapes and designs with special materials that absorb rather than reflect the energy in radar beams so enemy gunners cannot "see" and shoot down the bombers.

Earlier this month, President Reagan announced that as part of a major new program to strengthen the nation's nuclear arsenal, he wants to build 100 MX missiles and 100 B1 bombers. The B1 would begin replacing aging B52 bombers in 1986.

Reagan also announced his intention to pursue "a vigorous research and development program for an advanced technology bomber Stealth ." Stealth would become operational in the early 1990s and is meant to insure a continued U.S. ability to penetrate Soviet air defenses.

In keeping with Reagan's decision and the tight secrecy surrounding the project, the Pentagon statement issued yesterday, after the stock market closed, said that:

" . . . The Air Force announces that Northrop has been selected as the prime contractor to proceed with initial research and development on advanced bomber concepts. Key members of the team include Boeing, LTV-Vought and the General Electric Aircraft Engine Group. All details are classified. No further comment will be made."

Northrop is based in Hawthorne, Calif.

The Northrop contract, which company president Thomas V. Jones said "will have a material impact" on the firm, is only for initial development work. If experimental models perform as advertised, a subsequent production contract would be awarded. Yesterday's announcement would appear to identify what that production team would be if things go well.

Northrop, which builds the F5 jet fighter for export, has not built a modern bomber although its teammate, Boeing, built the B52 and others.

In congressional testimony, Pentagon officials have estimated that $9 billion to $10 billion may be needed just for "Stealth" development. Aviation Week magazine has estimated total development and production costs at $21.9 billion.

The cost of the project, coming atop a B1 bomber program unofficially estimated to cost about $25 billion, has made the dual bomber approach controversial in Congress.

Critics, including former defense secretary Harold Brown, argue that "Stealth" will be available within about five years after the B1 and that buying the B1 instead of extending the B52s' use and concentrating on "Stealth" is wasteful.

The administration argues that betting exclusively on the B52 much longer or on "Stealth's" technical and scheduling uncertainties is too risky.

Northrop and Lockheed have worked on "Stealth" and were widely reported to be competing for the bomber contract.

However, Aviation Week also has reported that Lockheed is developing a new fighter plane and cruise missile using "Stealth" technology under Pentagon contracts. That suggests that industrial participation remains widespread and that an array of Stealth-type weapons is under construction.

The magazine reported this week that Northrop's first test bomber is scheduled to make its first flight late in 1984 and that Lockheed's new fighter will fly this year.