 The B-17s made their first significant operational contribution during Joint Air Exercise No. 4, an Army-Navy summer maneuver conducted at sea west of San Francisco, California. Authorized by a directive of President Roosevelt issued 10 July 1937, the exercise used the target ship Utah to represent a hostile fleet of two battleships, an aircraft carrier, and nine destroyers. A patrol wing of 30 Navy planes commanded by Rear Admiral Ernest J. King was assigned to locate the fleet, after which a force of 41 Air Corps bombers (including seven B-17s operating from March Field) would attack it. War Department orders limited the Air Corps aircraft to operating no more than 300 nautical miles (560 km) offshore, even though the directive had specified an exercise area out to 500 nautical miles (930 km).
