The interception of the Rex was a training exercise and military aviation achievement of the United States Army Air Corps prior to World War II. The tracking and location of an ocean-going vessel by B-17 Flying Fortresses on 12 May 1938 was a major event in the development of a doctrine that led to a United States Air Force independent of the Army. The mission was ostensibly a training exercise for coastal defense of the United States, but was conceived by planners to be a well-publicized demonstration of the capabilities of "heavy bombers (as) long range instruments of power".