Overview of American aircraft development before and during World War II. In The Dragon's Teeth?, Kelsey observed that, for proper defense, a nation must maintain a "force in being", the same concept as 'fleet in being' but applied to the entire military of a nation at peace. Kelsey wrote that a nation must save its money and keep a core of military engineering and manufacturing industries alive by giving them enough business so that they don't disappear. In response to an attack, these industries could quickly expand to meet the challenge. Kelsey compared this careful husbanding of the potential for war-making effort with the myth of Cadmus, a Phoenician prince who supposedly sowed dragon's teeth in the ground to create an instant army. Kelsey flew as Lieutenant Jimmy Doolittle's safety pilot during the first fully 'blind' instrument flight on September 24, 1929. Kelsey flew the first prototype of the P-38 Lightning. In July 1942, he flew a P-38F[22] in the first ferry flight of fighters across the North Atlantic to England. In October 1954, Kelsey was a key member of the Research Airplane Committee which underwrote the North American X-15.