I read many times in English Wikipedia that some servicemen were trained in Flexible Gunnery schools, but I don't get what that exactly means.
In January 1942, the 2d Air Force was withdrawn from the Western Defense Command and assigned the operational training of units, crews, and replacements for bombardment, fighter, and reconnaissance operations. It received graduates from Army Air Forces Training Command flight schools; navigator training; flexible gunnery schools and various technical schools, organized them into newly activated combat groups and squadrons, and conducted operational unit training (OTU) and replacement training (RTU) to prepare groups and replacements for deployment overseas to combat theaters.