The Pentagon confirmed yesterday that it was working on a "Stealth" airplane that would be virtually invisible to enemy radar.

Pentagon spokesman Thomas B. Ross said that one reason the Carter administration was lifting the veil of secrecy surrounding the plane was that advances in the Stealth program are "part of Congress' consideration of a new manned bomber."

Also, said Ross, the program has reached the point where it will require much more money. While he did not say so, Ross indicated that Stealth was getting too big to hide in the Pentagon budget.

Skeptics at the Pentagon and in Congress theorized yesterday that the real reason the Pentagon was disclosing Stealth was to strengthen President Carter in rebutting charges that he has let down the nation's defenses.

Carter canceled the B1 bomber in 1977, a decision that the Republican presidential nominee, Ronald Reagan, and others have focused on in attacking administration defense policies.

The Stealth program, Ross said yesterday, "has been the subject of a very intensive administration effort since the beginning of the administration." He said news of the secret project had been leaking out to reporters, and the number of people being briefed on it had widened, increasing the chance for additional news leaks.

Given that situation, Ross said, the Pentagon will try to hold a public briefing on Stealth "hopefully in the next few days."

One congressional defense specialist theorized yesterday that election-year politics was the biggest reason for the Pentagon's sudden confirmation of Stealth.

"If they could keep it secret for over two years, they wouldn't have to go public now if it weren't for politics," he said.

In approving the fiscal 1981 weapons bill, the House and Senate Armed Services committees directed the Pentagon to choose a new bomber by March 15, 1981, and have it on duty by 1987.

However, congressional sources predicted yesterday that the advances in Stealth technology were so impressive that if Congress got a firm administration commitment on the bomber it might extend the 1987 deadline. Stealth could not be ready until the 1990s.

The Washington Post reported on Stealth Aug. 14, and said the breakthrough in technology "will enable Carter to argue that his cancellation [of the B1] was a good move because the contemplated new bomber could foil soviet defenses, which are becoming lethal enough to down a B1."