The export credit institutions of the United States, Italy and Japan have reached an unusual agreement to co-finance the next-generation Boeing 767 jet airplane.

Pending final confirmation by their governments, the Export-Import Bank of the United States and its Italian and Japanese counterparts signed a memorandum of understanding last week to provide financing for the 767 in direct proportion to their involvement in its manufacture.

The Boeing 767 is a drawing-board medium-range aircraft seating approximately 200 that will be produced by manufacturers situated in the three countries, with the United States accounting for the largest share. Its price tag is about $38 million on delivery in 1983.

The plane's major competitor is the Airbus A310, being produced by a consortium of aerospace companies in France, Great Britain, Germany, Spain, Belgium and the Netherlands, although a significant proportion of the plant, including the engines, may be supplied by U.S. components manufacturers.

Because the competing aircraft have generally similar characteristics and will both utilize the latest in aviation technology, John Moore, chairman of the Eximbank, said the new agreement may be instrumental in channeling some sales to Boeing. "Clearly financing is going to be a very important factor in determining which aircraft an airline may buy," he said.

"This is also a further indication that the United States is serious in meeting foreign competition," Moore said. "This administration will not see our markets go to foreign suppliers for lack of appropriate financing."

The Eximbank said the understanding will also result in a simplification of administrative procedures, allowing the buyer in many cases to negotiate only a single credit agreement instead of more complex multiple credit agreements in different languages and based on different financial systems. Eximbank would take the lead in processing the requests for assistance.

The U.S. and Italian partners are expected to provide their financing in dollars and the Japanese portion will be denominated in Japanese yen.