This monograph recounts the development of air doctrine in the Army air arm from 1917 to 1941. It includes concepts, both strategic and tactical, that emerged during World War I and the period following, up to the entry of the United States into World War II. The study is based primarily on official Air Force records and upon interviews with officers of the air arm who have been especially associated with air doctrine. It was prepared for the USAF Historical Division by Dr. Thomas H. Greer, formerly a member of the Division and presently associate professor of humanities, Michigan State College. A number of changes in, and additions to, the original draft have been made by members of the Historical Division, notably Mr. Robert T. Finney and Dr. Albert F. Simpson. All such changes and additions have been based upon Historical Division studies and data not available to Dr. Greer when he prepared the basic draft. Both the original draft and the final version were edited by personnel of the Division. Like other Historical Division studies, this history is subject to revision, and additional information or suggested corrections will be welcomed.