Pan American World Airways Inc. said yesterday that it has picked one of its executive vice presidents, William H. Waltrip, to run its airline business and is launching a search for a boss for the entire company.

And Pan Am Chairman William T. Seawell, who has run the company since 1972, said he will retire by the end of 1981. Seawell, 63, announced up a special committee to look for his sucessor.

Meanwhile, four major unions representing employes of the financially airling airline have pledged to "do whatever is necessary" to keep the airline going.

Officials of the unions -- representing Pan Am's pilots, teamsters, flight engineers, mechanics and office clerks -- are scheduled to meet with Seawell today to discuss "direct employe financial involvement" in the airline.

Their pledge -- and request for a voice in future Pan Am policy-making -- had comes amid increasing rumors that Pan Am's directors were close to decisions on a new president to succeed Dan A. Colussy, who left the company late last year, and a possible restructuring of the airline's route system to eliminate unprofitable routes. The recurring rumors also had included the possibility that Seawell's role at the airline might be diminished.

Pan Am has been suffering from the same combination of higher costs and downturn in passenger traffic as many other airlines. But its problems have been compounded by an enormous debt burden and a failure to achieve the benefits it expected to reap with its acquisition of a domestic route system through the pruchase -- some think at too great a cost -- of National Airlines.

Pan Am, which reported record losses of $114.5 million in the first quarter and $87.8 million in 1980, has taken some steps recently to reduce the number of its management employes and has asked employes of its five major unions to forgo about 10 percent of scheduled wage increases.

Officials of the Air Line Pilots Association, Flight Engineers International, International Brotherhood of Teamsters and Transportation Workers Union wrote Seawell last week that they recognize "the urgency of the corporation's economic situation and will do whatever is necessary to make it survive." (The union representing the airline's flight attendants wasn't a participant.)

The union leaders said they were prepared to enter into immediate negotiations to develop and support a plan they could recommend to their members. They said they envisioned "direct employ financial involvement" as well as "employe input into the decision-making process" in developing the airline's corporate philosophy and strategic opertaing plan.

Dick Phenneger, a Pan Am flight engineers official, said the letter was hand-delivered to Seawell last Thursday, and he responded by inviting them to meet with him Wednesday.

Waltrip, 44, has been Pan Am's executive vice president for marketing and planning, and has been with the airline since 1972. Under a corporate resturcturing, he became president and chief executive officer of the airline division yesterday.

"In today's environment, the problems of our airline division, which is by far the largest of our operations, require the undivided attention and efforts of a single executive head," Seawell said. "The directors have wisely chosen Mr. Waltrip to assume that responsibility."

Pan Am has not had a president since last November, when Colussy quit after repeated quarrels with Seawell. There had been speculation that Waltrip would replace him, but his new job leaves him without overall corporate authority.