Juan T. Trippe, 81, who developed a 90-mile airmail route between Key West, Fla., and Havana, Cuba, into Pan American World Airways, died Friday at his home in New York City. He had been ill since having a stroke last fall.

Mr. Trippe and two friends, Cornelius Vanderbilt Whitney and John T. Hambleton, founded an airline in 1927 with a German-made, Fokker three-engine monoplane. Besides the airplane, the company's chief asset was the first contract ever awarded by the U.S. government for overseas mail service. The inaugural flight was made on Oct. 28, the plane taking off from a dirt runway for a trip that took one hour.

Very soon, the Trippe-Whitney-Hambleton operation merged with a rival group, Pan American Airways Inc. Mr. Trippe became president and chief executive officer of the new company, positions he was to hold until he retired on May 7, 1968.

In the intervening years, Pan Am pioneered air service to Latin America, opened the first service across the Pacific in 1935, the first service across the Atlantic in 1939, and eventually, provided scheduled service to 85 countries on six continents. It was known as "the world's longest airline." To see that its passengers (and other wayfarers) would be comfortable when they got where they were going, it also operated hotels in many of the countries it served. The company's assets at the time Mr. Trippe stepped down were said to be more than $1 billion.

Unlike other pioneers of the aviation industry -- the flamboyant Capt. Eddie Rickenbacker is a case in point -- Mr. Trippe was shy of publicity. But he had no difficulty in making known views that he thought were important. These included low-cost fares to put air travel within the reach of the average person and the conviction that the U.S. aviation industry must help the emerging nations enter the age of flight. Pan Am was a leader in providing tourist as well as first-class air fares. It also led in "fly now, pay later" plans. In 1954, it began the first round-the-world air service.

Earlier, Mr. Trippe had championed the "chosen instrument" theory under which the government would designate a single U.S. air-carrier for all foreign flights. In fact, until the Civil Aeronautics Act was passed in 1938, Pan Am did enjoy such a status.

But when other airlines were authorized to compete in foreign markets, Pan Am still enjoyed a close relationship with Washington. During World War II, its facilities were on lease to the government. It took part in the airlift of troops to Korea in 1950. In Europe, it had as many as 64 daily flights in and out of West Berlin in the early 1960s when the Berlin Wall was built and it appeared as if the Cold War might reach a flashpoint and become something worse.

At the request of the government, Pan Am flew regular flights to Havana after Fidel Castro had taken control of Cuba. (They arrived there virtually empty and came back full of refugees). And the airline is the prime contractor for the Air Force at the Atlantic Missile Range based at Cape Canaveral, Fla.

A firm believer in American equipment and the role of commercial airlines in maintaining a strong national defense, Mr. Trippe was the first to order U.S. jetliners. He supported plans for this country to build a supersonic jetliner.

To a large extent, all of this was the work of a patrician of the Eastern Establishment whose interest in airplanes began in childhood, but whose first venture into the business world was selling bonds on Wall Street with a view to entering a family business there.

Juan Terry Trippe was born at Seabright, N.J., on June 27, 1899. He was a descendant of a family that had settled on the Eastern Shore of Maryland in the 17th century. He was named after a Spanish grandmother, Juanita. He grew up in New York City, where it is said that he made model airplanes powered by rubber bands. He prepared for Yale at the Hill School, where he was so quiet that classmates nicknamed him "The Mummy."

At Yale, he took private flying lessons and became a Navy aviator during World War I. When he finished Yale, where he founded the Yale Flying Club, he went to Wall Street. Early in 1923, he learned that the Navy was selling nine surplus planes. He and some colleagues from the Yale Flying Club bought seven of them for $500. With these they organized Long Island Airways Inc., which took sightseers up from Rockaway Beach.

A year later, he persuaded a group of Boston bankers to capitalize the Colonial Air Transport Company, which provided service between New York and Boston. The company got the first domestic airmail contract let by the government. Mr. Trippe pushed to expand the service to Chicago and Miami, thus gaining control of air service in the eastern United States. When stockholders balked at this seemingly grandiose idea, Mr. Trippe resigned.

His next venture became Pan American.

Mr. Trippe received the Medal of Merit from the United States for his services during World War II and numerous other honors from institutions in this country and from foreign governments.

Survivors include his wife, the former Elizabeth Stettinius, whom he married on June 16, 1928, of New York City (her brother, Edward R. Stettinius Jr., was U.S. secretary of state at the end of World War II); a daughter, Betsy Wainwright of New York City; three sons, Charles W., of Chicago, John T., of Doylestown, Pa., and Edward S., of Greenwich, Conn., and 12 grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to Yale University, New Haven, Conn., or to New York Hospital -- Cornell Medical Center, New York, N.Y.