The cost of the Air Force's new Stealth bomber has climbed to an estimated $450 million per plane, a 20 percent increase over past estimates, making it the most expensive bomber ever built, according to U.S. officials familiar with the latest cost figures submitted to Congress.

At the same time, the highly expensive Stealth aircraft, which already is flying, has been kept in production longer than planned under a secret deal engineered by Senate Armed Services Committee Chairman Sam Nunn, D-Ga., these officials added.The Stealth bomber is designed with a curvy shape and an absorbent skin, which should enable it to evade Soviet radars for a longer period than conventional aircraft, giving defenders less time to react and knock the invader down.

Officials said the Air Force has told a few senators and House members that its fleet of 132 Stealth bombers being built by the Northrop Corp. will cost at least 20 percent more than projected. The fleet's advertised price tag, in fiscal 1981 dollars, has already jumped from $36.6 billion to $43 billion, the officials said.

The only statement the Air Force has made about rising costs on its stealth bomber came April 20, when the service said it was "reevaluating cost estimates."