Death Valley Outlaws (1941)

"Ronning!...Killing!...Looting! Defied only by fearless Don Barry, the scappiest outlaw-wrecker in all of the west!"

Ambushed by the Vigilantes, a dying friend gets Johnny who was only passing through to take up the fight. To get in with the gang, Johnny poses as an outlaw and then beats them to a gold shipment by robbing the train ahead of them. This gets him invited into the gang. They are all masked and unknown to Johnny, one of them is his brother.