When the Seattle Chamber of Commerce raised the idea of a major airport in January 1941 and started surveying possible sites in King County. Even before Pearl Harbor, Boeing Field, named for the airplane manufacturer, was then King County’s main commercial airport. The growing demands on Boeing Field and the aviation hazards posed by the high ridge to its east, had already prompted calls for a new airport.

The Seattle Port Commission voted March 2, 1942, to build the new field on 907 acres of forest and scrubland at Bow Lake, midway between Seattle and Tacoma. The site selected for SEA Airport initially housed facilities for raising domestic rabbits, a frog patch, a mushroom farm, and a dog kennel. 

Work on the new airport began on January 2, 1943, and construction of the first runway was completed in 1944. And the ceremonial first landing was by a United Air Lines DC-3.  The first transcontinental passenger flight from SEA to New York was flown by Northwest Airlines in a DC-3 which took 17 hours.

Pan Am signed the first lease to build an airline terminal and hangar, but the airport saw little commercial use until 1946. Rather, the Army Air Force used the new airport for transshipment of thousands of Boeing B-29 bombers after the war.

https://www.portseattle.org/blog/1940-1949-birth-seattle-tacoma-international-airport

https://www.historylink.org/File/1004