Northrop Corp. yesterday made an unusual bid to break into the Air Force jet fighter market, offering to sell almost 400 F20s to the U.S. government at what it said was a substantially lower price than that of the F16 produced by General Dynamics Corp.

Northrop, which developed the F20 Tigershark for overseas sales but has yet to sell any, said acceptance of its offer would introduce true competition into Air Force purchases of jet fighters for the first time in years.

Thomas V. Jones, chairman and chief executive officer of the California-based company, also offered what he called an "unprecedented" guarantee that prices for spare parts would not change during the next 20 years.

Both the fixed-price spares and the promise of competition are designed to appeal to a Defense Department buffeted by contractor and billing scandals, particularly in sole-source deals.

The offer came at a difficult time for General Dynamics, which is the nation's largest military contractor but has become the object of a dozen investigations into allegations of fraud and mischarging.

"Northrop believes this is the kind of initiative from private industry that the Air Force has been looking for and that the Air Force and the country deserve," Jones said in a statement after his offer was delivered to Air Force Secretary Verne Orr and Gen. Charles A. Gabriel, the Air Force chief of staff.

The Air Force has praised the F20 in the past but has said it would not meet U.S. needs.

Orr yesterday continued to sound skeptical, but did not rule out altogether a purchase of F20s.

"With some committees of Congress nearing completion of their markup, it would be very late to attempt any alteration of the budget for the coming year," Orr said in a statement. "The Air Force has no intention of asking for any change which would meet Northrop's Dec. 1, 1985, deadline.

"However, preparation of the 1987 budget is under way, and discussions within the Defense Resources Board will commence in July," Orr added. "The advantages of competition versus the disadvantages of procuring yet another fighter aircraft will be discussed at that time."

The Air Force has bought about 800 F16 fighters and hopes to buy about 2,000 more at a rate of about 180 a year. The service also is buying F15s, which are made by McDonnell Douglas Corp. and cost more than $40 million each.

The F16 costs between $18 million and $20 million, depending on how costs are calculated. Northrop said it would sell its F20s for $15 million each.

Northrop also said that it would provide spare parts for its planes at a rate of $475 per flying hour. If the planes turned out to require more maintenance than the company expects, Jones said, the company would pay the difference.

Orr introduced competition into Air Force purchases of jet fighter engines and said that it led to reduced prices and increased responsiveness from both companies involved.

At the same time, the Air Force is being criticized in Congress for buying too many different types of aircraft, each of which demands different tools, parts and training.