Call a Messenger (movie, 1939)

A tough street kid attempts to rob a post office and is caught. In order to avoid reform school, he takes a job as a messenger with the post office. He finds that he likes it, and when his brother is released from prison, attempts to help his brother go straight. However, the two of them get mixed up with a local gangster, who has plans to start robbing post office branches and using the messenger and his brother to do it.