United Airlines, shifting its attention from the buyout bids that have preoccupied it for three years, yesterday announced it is placing orders and options for up to 128 Boeing Co. jets potentially worth more than $22 billion.

If all the options are exercised, it would be the biggest aircraft order in history.

The order marked several turning points, including the first firm order for Boeing's new 777 jetliner, a twin-engine plane that is slightly smaller than a 747 jumbo jet. The most visible change from previous models will be the folding wingtips that will allow the jet to use airport gates now used by mid-sized planes.

Boeing had been courting a number of airlines for the 777 without getting any firm orders until yesterday. In addition to United, other airlines are expected to sign on for the 360-seat aircraft, possibly including All Nippon Airways, Delta Air Lines, American Airlines and British Airways.

The announcement also substantially improved the outlook for Boeing, which had been worrying that demand for aircraft might be softening. It brings total orders for the year close to $40 billion, according to industry analyst Paul H. Nisbet of Prudential-Bache Securities Inc. As a result, Boeing may surpass last year's record of $46.7 billion in aircraft orders by the end of 1990, despite the dismal state of the airline industry, Nisbet said.

United's order, which included a firm commitment for $11 billion worth of aircraft, includes 34 of the new 777s. It also includes firm orders for 30 Boeing 747-400s and options for an additional 34 777s and 30 747-400s. "The importance of today's commitment for Boeing cannot be overstated," said Boeing Chairman Frank Shrontz.

The past several years have produced a bonanza of orders for aircraft manufacturers as airlines equipped themselves for traffic growth. But the downturn in the airline industry caused by soft demand and high costs for jet fuel have made airlines more cautious. On the other hand, high fuel costs often argue for replacing older, less fuel-efficient aircraft with newer models.

United had deferred the order while a buyout bid by its employees was pending, allowing the employee group to negotiate with the aircraft manufacturer and engine makers over potential financing for the bid.

Last week, however, after financing failed to materialize for a bid that United's parent, UAL Corp., had accepted, UAL's board let the buyout agreement expire and said the airline would return to business as usual. Yesterday's order underlined that intent and also suggested "that the airline is moving rapidly to preclude any thoughts of {a leveraged buyout} group or stockholders to use the money that is being spent for the aircraft," said Nisbet.

"They're refocusing on the fundamental growth of the airline and trying to get away from the buyout mentality of the past couple of years," said Mark Daugherty of Dean Witter Reynolds Inc. With the order, United may be well-positioned when the economy recovers from its current downturn, he said.