This study examines the development and usefulness of US air attack theory and doctrine during the interwar period, 1919-1941. This period represents more than twenty years of development in US Air Corps attack theory and doctrine. It was the first peacetime period of such development. Attack aviation during this time was a branch of aviation used to provide direct and indirect combat support to ground forces in the form of machine gun strafing, light bombing, and chemical attacks. From the earliest origins, attack theory and doctrine evolved primarily along two paths--- direct and indirect support of ground and air force objectives. The direct support approach was based on fundamental beliefs by the Army that attack aviation was an auxiliary combat arm, to be used directly on the battlefield against ground forces and to further the ground campaign plan. The indirect support approach, or air interdiction, was derived from the fundamental beliefs by the Air Corps that attack aviation was best used beyond the battle line and artillery range, against targets more vulnerable and less heavily defended, to further both the Air Force mission and the ground support mission. The Air Corps Tactical School advocated the indirect support approach and the subsequent evolution and logic in attack doctrine flowed from this approach. Air Corps theory and doctrine called for attack aviation to be used beyond the battle line. Aircraft were less vulnerable to ground fire and could be used to delay and disrupt enemy ground forces. Less cooperation was required with the ground forces while more cooperation was needed with other aviation branches, especially pursuit aviation. As attack doctrine evolved, range and hardened targets became problematic for the single-engine attack plane. The indirect support approach, supporting both the Air Force and Army missions, required an aircraft with increased range and payload. Subsequently, the attack-bomber, or light bomber was introduced to meet the attack requirement. What appeared to be neglect, and the overly strong influence of strategic bombing doctrine, was more accurately, an evolution in the development of attack aviation doctrine. Thus, attack theory and doctrine in terms of the indirect support approach, was adequately developed to be useful at the start of WWII. The use of light and medium bombers in North Africa showed the effectiveness of air interdiction and the indirect approach. Attack aviation had, indeed, established itself before WWII. Attack aviation, in the form of close air support, would have to wait for the lessons of WWII.