The B-17G was the result of an almost continuous improvement program of earlier B-17 models. The -G model was basically the production version of the B-17F after the modifications and improvements were incorporated into the design. Although the Bendix chin turret is the most obvious improvement incorporated into the B-17G, it was actually first used on late model B-17Fs. More than 8,500 -Gs were built by three different manufacturers: Boeing, Douglas, and Lockheed-Vega. More than 12,500 B-17s of all types were built before production ended.