McDonnell Douglas Corp. said yesterday that Alitalia, Italy's national airline, has agreed to buy 30 DC9 Super 80 twin-engine jets in a transaction valued at more than $1 billion.

The sale, contingent on approval by the Italian government and on satisfactory financial arrangements with the U.S. Export-Import Bank, is the largest order to date for the stretched DC9, a medium-range aircraft that can seat up to 172 passengers.

Yesterday's announcment was the third in six weeks for sales of the Super 80. McDonnell Douglas recently signed agreements with American Airlines and Trans World Airlines, under which the two will lease a total of 35 of the aircraft for five years. The leases and the sale allow the aircraft maker to continue to maintain its current production schedule of three a month.

The Alitalia purchase was also a blow to Boeing Co., which had hoped to sell its new, $32 million twin-engine 757s to Alitalia. Boeing had flown a 757 to Rome from England's Farnborough Air Show in September to demonstrate the plane to Alitalia and Italian government officials.

A Boeing spokesman said yesterday the company was "disappointed, of course. . . . But we'll be back with another sale somewhere else."

The aircraft, which cost about $22.5 million each, will be powered by two Pratt & Whitney engines, which provide more thrust than earlier versions but burn less fuel and are quieter.

McDonnell Douglas has delivered 91 of the Super 80 so far, with PSA Inc., a California-based airline, the largest user, with 21 of the aircraft.