After the problems with the Colt XM-148 under barrel grenade launcher were discovered in service in Vietnam, the US military was eager to quickly find a better alternative. The idea of replacing the standalone M79 grenade launcher with an under barrel unit was very appealing, and the development program moved quickly. Of seven different designs considered, AAI was the best, and it was adopted as the M203 and put into the field by 1969.
The M203 was a dramatic improvement over the XM148, with much simpler controls and batter handling. It would be made in large quantity (over a quarter million) and used until replaced only recently by the HK 320 launcher.