The Navy's $6.2 billion program to design the A-12 "stealth" bomber was conceived during the height of the Cold War, when the Soviet Union still was considered an "evil empire" that threatened America's security.

But it died in the entirely new era of Mikhail Gorbachev's perestroika, with Washington and Moscow cooperating in the Persian Gulf, the Pentagon wildly scrambling to find a new mission for its sophisticated weapons and Congress pressing for a peace dividend from the end of the Soviet threat.

General Dynamics Corp. and McDonnell Douglas Corp., the two developers of the A-12, have painfully discovered what others may soon learn from Defense Secretary Richard B. Cheney's decision Monday to cancel the Navy's highest priority aircraft: America's defense industry faces a troubled future as the Pentagon adjusts its budget and its mission to the post-Cold War era.

Even projects once thought to be invulnerable are at risk as the nation's defense priorities undergo what could be a long review.

"With the decline of the Soviet threat, we can live without it {the A-12}. To a certain extent, it is a victim of the Cold War," said Lawrence J. Korb, a former assistant secretary of defense who now is a senior fellow at the Brookings Institution.

Former Navy secretary John F. Lehman Jr., now a New York investment banker with Paine Webber Inc., said both the A-12 and the troubled Air Force B-2 "stealth" bomber programs were conceived to beat future generations of Soviet air defenses.

"The question is whether the Soviets are going to put that kind of money into developing a new air defense system. If they don't," asked Lehman, "who will generate the threat" to require the expense of developing "stealth" bombers?

"My answer would be the prudent planner would not say the Soviets are going to make that kind of effort to warrant the A-12."

Wall Street analysts predict that more weapons development programs will bite the dust as the Pentagon budget crunch forces military planners to take a closer look at their cost and usefulness. Cheney said he canceled the A-12 after contractors and Navy officials could not tell him how much it would cost to correct major problems in the plane's development.

"The budget will not be large enough to permit the development of newer weapons as well as production of existing systems," said Lawrence M. Harris, aerospace industry analyst at the Los Angeles investment firm of Bateman Eichler, Hill Richards.

Stock in McDonnell Douglas, which faces problems in its commercial aviation division as well, plummeted $7.62 1/2 to $31.25 yesterday after the New York Stock Exchange delayed its trading at the opening. General Dynamics stock dropped $2.50 to $22.50. The two leading bond rating agencies, Moody's Investor Services Inc. and Standard & Poor's Corp., also announced yesterday that they were reviewing the creditworthiness of both companies.

General Dynamics and McDonnell Douglas responded to the A-12 cancelation by denying Cheney's claim that they had defaulted on the contract and by saying they would lay off close to 8,000 workers, largely engineers who were designing and building prototypes of the sophisticated plane. The A-12 ran into trouble meeting the stiff Navy requirements of being able to withstand the "controlled crash" shocks of carrier landings and takeoffs while possessing the "stealth" capability to slip past enemy radar.

Underscoring the new era facing defense contractors, Pentagon spokesman Pete Williams said yesterday the government will try to collect $1.9 billion from General Dynamics and McDonnell Douglas for work paid for but not delivered. "We've paid them money, we've gotten some goods and services, {and} we'll go back and try to get the rest of the money back," said Williams.

The two contractors denied they owe the government anything and said they will fight the government claims. The government has paid $3.1 billion to the two companies for the program, which has been plagued by charges of mismanagement that contributed to the resignation of the Pentagon's senior procurement officer and led to the firing of two admirals and a captain. The Justice Department also has opened a criminal investigation of the payments to the two companies.

But the program was troubled for other reasons. The Navy originally had planned to buy 620 of the sleek new A-12s. But as the Pentagon began looking ahead to an era with no major enemy, along with the need to fight different kinds of wars in places like Grenada, Panama and Iraq, it reduced its orders. That made it harder to hide some of the cost overruns of development in the prices charged for the finished planes.

"If the budget was $10 billion to $15 billion larger, the Navy could have swept this under the rug. You can't hide $1 billion easily anymore," said an industry official.

Furthermore, in seeking state-of-the-art technology, the Pentagon may have ordered a plane that was too expensive for the new environment. "Decisions that were rational five years ago don't look as good now," an industry official said.

"What you are seeing here is the market telling you that the price of these kind of airplanes is more than the money available in the budget to pay for it. The money is just not enough to pay for that degree of capability," said Lehman.

He said the Navy could get away with upgrading the current 40-year-old designed of the A-6 attack bomber with the advanced radar and avionics that have been bought and paid for in the development of the A-12. He asserted this will be enough to keep the United States ahead of air defense systems thrown up by potential enemies.

As evidence of this, he said, the A-6 "was able to slam dunk the current state-of-the-art Soviet-French air defense systems" when they bombed Libya five years ago.