Plans to build a 150-passenger jetliner probably will not be realized before 1990 because the engine for a such a plane has yet to be developed and the market is uncertain, top executives of McDonnell Douglas Corp. said this week.

Sanford N. McDonnell, chairman and chief executive officer of McDonnell Douglas, and James E. Worsham, president of the subsidiary Douglas Aircraft Co., were in Washington this week to announce the sale of 30 new DC9 Super 80s to Alitalia, Italy's national airline. The order, previously reported, is valued by McDonnell Douglas at more than $1 billion and is the largest commercial transaction in the company's history.

The prediction about the 150-seat, two-engine jetliner came during an interview. That aircraft, which Delta and USAir have said will be needed, has been designated the D3300 at Douglas, "and we are aggressively continuing work" on its development, Worsham said.

McDonnell said, however, that "we feel it would be a mistake to get out an airframe and then have to retrofit the frame to a new engine as Airbus the European manufacturer is doing. We would lose the technology gain" that comes from developing the airframe after knowing the specifics of the engines, he said.

McDonnell and Worsham were obviously pleased about the sale to Alitalia. It came after lengthy negotiations and serious competition from Airbus and gives Douglas 85 firm orders for new Super 80s and conditional options on 46. A total of 101 Super 80s have been delivered to European airlines and several U.S. airlines that fly in the West.

A year ago, some members of the aviation community feared that McDonnell Douglas, a merger of two of the grandest names in flying, would have to abandon the commercial transport business. Douglas' new offering, the Super 80, was not selling because of the airline depression. The DC10 was caught in a market that has dozens of used jumbos for sale and little room for new ones.

Worsham conceived leasing arrangements that made it possible for Douglas to put 35 new Super 80s in the hands of American and Trans World airlines. Now the Alitalia sale gives Douglas more solid orders to keep the production line running.

"We outsold, both in numbers and in dollars, Boeing and, by a country mile, Airbus," in 1982, McDonnell said.

The Alitalia order, helped by a preliminary commitment for financing from the Export-Import Bank, will provide more than 14 million man hours of work in Italy for Aeritalia, a firm that has produced fuselage panels and other structures for the all DC9s built over the past 14 years.

Douglas is planning two more versions of the Super 80, which will give it an airplane "family" similar to the one that has made Boeing such a tough competitor in the jetliner market, McDonnell said.

At the same time, the Air Force has fallen in love with the KC10 tanker, a derivative of the DC10, and has ordered 60 of them; 12 have been delivered under a multiyear contract.

That keeps the DC10 line open. At the same time, Douglas is continuing to develop a follow-on aircraft, the MD100, with a new cockpit and engines. However, "The market is just not ready," Worsham said.

For the first nine months of 1982, McDonnell Douglas showed sales of $5.4 billion, up slightly from $5.29 billion for the same period a year earlier.