The film was made in 1950, and this was an important film. What he meant by that was that it was a turnaround in the public's attitude toward his character. When he came back from the war in 1945, the world had changed a lot.
Nobody goes through that kind of trauma without being changed, neither a nation nor an individual.
As a bomber pilot, he flew B-24's for the 8th Air Force, eventually becoming a squadron commander. That's something he never forgot. It's kind of simple. Let's make a movie about that. Obviously, it's not about that, but it's just such a different character for Jimmy Stewart to play.