The Man behind Pan Am; The Chosen Instrument, by Marylin Bender and Selig Altschul. New York: Simon & Schuster. 605 pp. $19.95.

''Pan Am was the American flag, for all practical purposes an extension of the United States government. In many places, it was the only symbol of America besides the embassy. Even before there was Coca-Cola in some places, there was Pan Am.''

This reminiscence by a former CIA officer, quoted in ''The Chosen Instrument, '' is one Pan Am's founder would have endorsed. From the airline's earliest days , Juan Terry Trippe strove to maintain an intimate relationship with the US State Department, which culminated in quasi-military operations during World War II.

With the help of this tacit partnership, Trippe succeeded in establishing a company renowned for its many ''firsts'': the first commerical airline to link the capitals of Latin America, the first to cross the Pacific, the first to buy jet aircraft, the first whose routes encircled the globe.

Even so, Trippe failed in his ultimate ambition: to enshrine Pan Am as the ''chosen instrument'' of the US government - the equivalent of a national airline, the only one appointed to fly overseas.

Trippe's personal strengths were largely responsible for Pan Am's triumphs. His weaknesses helped bring about its current semicollapse. The life histories of man and airline are inseparable, and Bender and Altschul cover both in ample detail.

Like a passenger on a transoceanic flight, the reader will encounter some dull stretches in this biography. But that doesn't alter the basic point: It flies.