President Carter, in a major arms policy reversal, has decided to let American defense contractors design and build warplanes specifically for foreign countries.

The State Department yesterday announced the change in the 1977 policy which restricted foreign arms sales to weapons designed for U.S. forces.

Carter, as part of his effort to curb the flow of modern weapons all around the world, imposed that restriction shortly after taking office.

With the restriction lifted, U.S. aerospace companies will scramble for the billions of dollars several foreign countries appear willing to spend for planes less sophisticated than the Air Force F15 but hotter that the F5 already available.

In anticipation of a change in policy, several aerospace companies already have designed planes expressly for export. The companies include General Dynamics, manufacturer of the Air Force F16 fighter, and Northrop, builder of the F5, sold to many countries around the world.

General Dynamics, offering is a longer version of the F16. It would be powered by a less-sophisticated engine than the one in that Air Force version. Northrop has designed a single engine version of its twin-engine F5. It had hoped to sell that plane, the F5G, to Taiwan.

Other companies are expected to enter the competition for sales, of an estimated 500 "FX" fighters abroad. This would mean over $5 billion in new business.

Under the Carter administration liberalized policy, hammered out before the Soviet invasion of Afghanistan, the companies, not the U.S. government, would finance the foreign aircraft sales.

General Dynamics has said it would build and deliver a modified F16 in two years. Northrop probably could respond in the same time frame.

Carter's new policy may make it more difficult for the administration to restrain other nations from selling warplanes abroad. The administration stopped Israel from selling its Kfir fighter to Ecuador on the grounds that such exports would accelerate the arms race in that part of the world. The Kfir is powered by an American engine, which gave the U.S. government a veto over such third-country sales.

Potential customers for the FX include Brazil, Jordan, Kenya, Saudi Arabia, North Yemen and Taiwan. Sales of the F5E have blazed the trail for the FX in those countries.

Administration officials, in defending the change in policy, reasoned that an intermediate fighter like the FX would reduce the appetite of nations for even more lethal planes, like the F15 and F16. They added that other nations have shown little restraint in their own warplane sales abroad.

During his presidential campaign, Carter deplored the world trafficking of arms, declaring that the United States could not be "both the world's leading champion of peace and the world's leading supplier of the weapons of the war."