James Stewart took trombone lessons in order to actually play during scenes where Glenn Miller is seen performing with his orchestra. However, while learning from his tutor, Stewart's attempts at practicing the instrument made such horrible noises that the teacher reportedly went home in a rage every night to his family. It was decided that Stewart would accurately mime the motions of playing (especially apparent during the performance of String of Pearls) while his trombone tutor Joe Yukl did the actual playing.
One of Glenn Miller's trombones is on display at the National Museum of the US Air Force in Dayton, OH, along with other Miller memorabilia. According to the display notes, the Miller estate allowed James Stewart to "play" this trombone in this film. Another trombone and additional memorabilia are displayed at the Glenn Miller Birthplace Museum in Clarinda, IA.
Among those attending the premiere of the film was Glenn Miller's mother, who thought the film good enough, but opined that her son was better-looking than James Stewart.
At the time of filming, James Stewart was five years older than Glenn Miller was when he died, which also meant that he was 45 when portraying the young Miller in his early twenties. June Allyson was 37 when playing the 22-year-old Helen.
Due to his profit participation arrangement for this film, James Stewart supposedly earned over $1,000,000 from this over the years.