The Vietnam War heightened the need for a dedicated counter-insurgency (COIN) aircraft. The answer to this became the Cessna A-37 Dragonfly, basically a conversion of their successful T-37 jet trainer for light attack duties against targets which would have been much more difficult for faster jets. Compared to the trainer, the A-37 had an internal gun, eight hardpoints, armor protection, and wingtip fuel tanks for greater endurance. After an evaluation period, A-37s began operating with US and South Vietnamese units where they were used for close support, helicopter escort, forward air control, and night interdiction carrying a wide array of ordinance. Eventually a dozen other countries adopted the A-37, mostly in Latin America for use against internal insurgencies but also Cambodia, South Korea, and Thailand. It would also go on to inspire numerous simliar designs such as the BAC Strikemaster among others.

The first converted YAT-37D flew on 22 October 1963 and was an armed version of the T-37B jet trainer. They were eventually designated A-37A followed by the improved A-37B with more powerful engines, re-stressed structures, increased fuel capacity and the ability to be refueled in flight. Some of these were modified for forward air control duties as the OA-37B while a lightly armed version of the T-37 trainer became known as the T-37C.