In 1942 Boeing committed half a million dollars to construct and equip a B-17 Flying Fortress training school on the north end of Boeing Field. It included barracks, three hangars, classrooms, a dispensary and a theater. B-29 training began in February 1943. The school ran day and night until the war ended and closed in October 1945. By then Boeing had trained 30,000 mechanics and flight engineers as well as pilots and navigators on how to maintain Boeing B-17 and B-29 bombers.

Along with the basic necessities of the school, Boeing added an innovative training aid: a two-story blockhouse with a pair of wings attached so the mechanics could go through their actual daily maintenance routine. The B-29 test rig was called "Unknown Angel."