Hail the Conquering Hero (1944)

Like his war-hero father who was killed in action during World War I, Woodrow Lafayette Pershing Truesmith (Eddie Bracken) leaves his small town to join the Marines during World War II. But when he's given a medical discharge after only a month, he takes a job at a San Diego shipyard and writes letters to his mother about his fictitious wartime exploits. When he returns home, he discovers that his innocent lies have spread further than he expected.