THE NORTHROP CORP. has come to town with a new product this year. It is offering the Defense Department and Congress its F20 fighter not just as an airplane but as an instrument of defense reform.

The F20 is an orphan, a not-quite top-of-the-line fighter that was developed with Carter administration encouragement beginning in 1980 for sale abroad. At least in its earliest form, it was a compromise, a way of satisfying the desire of foreign governments for modern arms without giving them the most dangerous and advanced ones. Partly because they understood that, no governments bought any, and the Reagan administration then confounded Northrop by offering some countries the Pentagon's somewhat more elaborate F16 instead. The more F16s that are bought, the lower that plane's unit cost.

Northrop's response has been to shift its target; it now wants to sell F20's to the Air Force. The Air Force already has two fighter planes: the fancy F15 and workhorse F16. It is not clear it needs a third. Defense Secretary Caspar W. Weinberger said last year the F20 was not good enough for U.S. forces. Northrop, however, has aggressively cast its offer to appeal not just to the military but the military reformers on the Hill.

A favorite reform tenet has it that too many weapons are gold-plated, that in many cases less would be more. Northrop offers the F20 not as superior to the F16, which it would displace in part, but as "comparable" and likely to be more reliable and less costly to maintain. The reformers oppose sole-source contracting; Northrop puts itself forward as competition for General Dynamics, builder of the F16. The reformers object to the cost-plus nature of so much procurement. Northrop says it will sell the Air Force 396 F20s over the next four years at the fixed price of $15 million each in 1986 dollars (as against $18 million to $20 million now for an F16). The California company also offers what it calls a fixed price on all spare parts needed over the expected 20-year life of its airplanes.

It is attractive to have contractors bidding downward for a change. There doubtless are missions the F20 can perform as well as the F16, and the Air National Guard, whose first mission is air defense, has urged the Air Force to buy some. The House Armed Services Committee, in its version of next year's defense authorization bill, has ordered the Air Force to "compete" the F20 and F16. The Air Force, meanwhile, has made room in its new five-year procurement plan for 126 F20s. Northrop's blitz seems to have produced the intended result.

That may also be the right result -- we do not know -- but we suggest caution. The F16 is a good airplane, and we remember earlier periods of defense reform when the orthodox view was that the services should consolidate their weaponry and have fewer kinds, not more. Mr. Weinberger warned last year against the support costs "associated with introducing a new aircraft into our inventory." Congress, in a time of rising defense costs, has a special burden to be sure it is buying the substance of reform, not just the label.