Northrop Corp., already facing investigations by at least two federal grand juries over its handling of major Air Force weapons programs, is under new scrutiny as a result of allegations that it overbilled the Pentagon by hundreds of millions of dollars on its largest and most sensitive program, the top-secret Stealth bomber.

The allegations, contained in a $2 billion false claims lawsuit filed by four current and former Northrop employees in Los Angeles, include charges that the Los Angeles-based aerospace firm destroyed internal evidence documenting at least $400 million in overbillings, falsified documents submitted to congressional investigators and paid high-priced engineers to do little or no work.

While the Air Force confirmed yesterday that it has launched an investigation into the charges, many Wall Street analysts questioned whether such large-scale fraud could have taken place. Nevertheless, some said, the charges represent a new blow to the company, fueling public perception that Northrop may be on the verge of becoming what General Dynamics Corp. was several years ago -- the defense contractor everyone likes to beat up on.

"The company is going to go through a rather miserable time -- they're going through something similar to what General Dynamics did," said Paul Nisbet, an analyst with Prudential-Bache Securities in New York.

The perception of Northrop as a company in trouble stems in part from highly publicized charges that company officials falsified test data relating to the internal guidance systems of the MX missile. Evidence of defective Northrop guidance systems has prompted the Air Force to withhold $130 million in progress payments to the company and has triggered a criminal investigation by the Justice Department.

Northrop has also been plagued by production and schedule problems on the Stealth and was forced to take about $214 million in Stealth-related writeoffs last year. As a result of these problems, the Air Force has delayed the date on which the Stealth will become operational and has even considered taking the management of the program away from the company, according to congressional sources.

"We continue to have concerns about that program," said House Armed Services Committee chairman Rep. Les Aspin (D-Wis.) yesterday about Northrop's management of the Stealth. "We're concerned about the cost, the schedule and the performance."

But Aspin stressed that the criticism that he and others have leveled at Northrop in the past are different than the specific allegations of fraud raised in the suit. "The rap on Northrop is in production ... their ability to run a production line," said Aspin. "This {the lawsuit} is a whole new set of charges and we can't factor them in until they are investigated."

Further complicating Northrop's problems is that the firm is prohibited from defending itself, at least in the public arena. Although the Stealth, sometimes called the B-2, accounts for more than 45 percent of its $6 billion in revenue last year, the bomber is still classified as "black" or top secret by the Air Force. Because of that, Northrop is barred from commenting on any aspect of the program.

The company's difficulty was painfully evident yesterday. The two lawyers who filed the suit -- flamboyant California attorney Herbert Hafif and his partner Robert Kilborne -- called a press conference in Los Angeles and attracted a swirl of national publicity as they accused Northrop and its management of bilking the taxpayers.

"What we're talking about here is one of the biggest thefts in history," said Kilborne, who with Hafif has filed two other civil suits against Northrop.

Among Kilborne's specific charges was that Northrop was billing the Pentagon for the work of "hundreds" of employees at its Pico Rivera, Calif., production plant who "sit around all day reading newspapers and drinking coffee." He also said that, within the past few days, Northrop's security department had contacted one of the plaintiffs and, in an apparent attempt at intimidation, threatened to prosecute him for violating federal laws governing the release of classified information.

Perhaps most significantly, Kilborne said that he and private investigators his law firm has retained have evidence that Northrop auditors last summer presented a draft audit that documented $400 million in overcharges at the Pico Rivera plant and extrapolated that there may have been up to $1 billion in overcharges through the course of the program.

But, according to Kilborne, Northrop management ordered the destruction of the audit and all of the related work papers. "All the papers were shredded and burned," Kilborne said.

It is difficult to evaluate these and other charges because the names of the four plaintiffs, and other witnesses who Kilborne says can document the allegations, have not been released. In addition, under new provisions of the False Claims Act -- enacted by Congress to help root out fraud against the government -- the lawsuit itself must remain under court seal for 60 days while the Justice Department evaluates the allegations.

If Justice determines there is merit to the suit, it can take over the case and assume the costs of litigating it.