What Jimmy Stewart movie? He was in prison for murder. But came up with ideas that revolutionized guns for ww2?

It was a movie where he was in an old school prison for a shooting he may or may not have committed. He was a good man who didn't want those who cared about him to care anymore, and gave up on life. And the only thing that saved him was keeping himself occupied thinking about ways to improve guns. He became good friends with the warden (who was very cool) and he was allowed to invent and test a new machine gun that revolutionized machine guns. And ultimately was pardoned for his ideas. Any clues? Its been bugging me for years.