William M. Allen, 85, who in 23 years as president of the Boeing Co. built it into the world's leading maker of commercial jet airplanes, died Oct. 29 at his home here. He had Alzheimer's disease.

Mr. Allen was "one of the great figures in modern American industry," said T.A. Wilson, chairman of the board and Boeing chief executive officer. "His courage and foresight led America into the jet age and to dominance in the jet transport marketplace."

The day Mr. Allen was named president of the company in September 1945, the government canceled orders for 5,000 B29 heavy bombers because World War II had just ended. Faced with the loss of military contracts, no civilian business and a bitter strike, Mr. Allen directed Boeing's reentry into the civilian aircraft business.

One of his biggest gambles was a decision to risk $16 million on a prototype of a commercial jetliner. The result of his willingness to "bet the company" on the new design was the 707, the first of the U.S. jets that eventually made Boeing the world's leading manufacturer of civilian passenger aircraft.

Mr. Allen said the decision to build the 707 began when he stopped in Wichita, Kan., to ride in the B47, a Boeing military plane with swept wings and jet engines. He said the ride was so superior that he knew Boeing had something. During Mr. Allen's tenure Boeing developed not only the 707, but also the 727, the 737 and the 747 jetliners. These made it the leading producer of commercial aircraft in the world.

Boeing had become a great name in American industry during the war. More than 200 airlines around the world use Boeing planes today.

Mr. Allen was born in Lolo, Mont., and went to Seattle to work as a lawyer after graduation from the University of Montana and Harvard Law School. He was assigned to do legal work for Boeing.

When the company won a government contract for transcontinental mail service, he drew up the legal papers for the formation of Boeing Air Transport. The subsidiary was split from Boeing in an antitrust action in 1934 and became United Airlines. Early in World War II, he drafted a contract between Boeing and the government that called for payment on the basis of cost plus a fixed fee. It became a model for much of industry during the war effort.

Mr. Allen became a Boeing director in 1931. He was named company president after the death in 1945 of P.G. Johnson. He held the presidency until 1968, when he was succeeded by Wilson. For the next four years he was chairman of the board and chief executive officer. He retired in September 1972.

By his own account, Mr. Allen was not an obvious choice to head the company. He had never taken a management course and lacked an engineering background. But he found a way to turn that to his advantage. "I was incompetent," he once said. "I told them so and they knew it was true. I told them that each of them was going to have to put out a little bit more for that reason. And they did."

A Time magazine cover story on Mr. Allen in 1954 gave these reasons for his success: "He knew when to gamble. He trusted his designers. He knew how to forge a solid team."

When other American and foreign companies followed Boeing into the commercial jet airplane business, Mr. Allen became the company's leading salesman, establishing business relationships with airline executives all over the world.

In 1970, on behalf of his colleagues, Mr. Allen received the Collier Trophy, the top award in the aviation industry, for development of the B52 bomber in 1955 and the 747 jumbo jet in 1970.

In 1927, Mr. Allen married Dorothy Dixon, daughter of the governor of Montana. She died in 1943.

Mr. Allen's survivors include his wife, the former Mary Ellen Agen, whom he married in 1948; two daughters by his first marriage, Mrs. Grant Silvernale of Kirkland, Wash., and Mrs. Nathaniel Penrose of Seattle, and two stepchildren, Mrs. Charles Stattuck Jr. of Portland, ORE., and James Agen of Burlington, Wash.