The Yanks Are Coming (1942)

Popular movie crooner Bob Reynolds (William Roberts) is drafted into the U.S. Army and not too happy about it. He is followed by Gil Whitman (Henry King) and his orchestra, who are just there to play the music when needed. There they carry on their musical activities as they prepare a show that takes place in the camp canteen the night that orders come for the corps to move into active service overseas. Before that, an army sergeant adjusts Bob's whiny attitude.