Shot in only eight days.
Actor Dick Miller, a Roger Corman regular, was scheduled to play the title role, but writer R. Wright Campbell kept pushing for his brother, William Campbell, to get it and began tailoring the script to emphasize his brother's strengths. To avoid internal squabbling, Corman assigned the role to Charles Bronson.
George Kelly was placed on the #1 spot on the FBI's "Most Wanted" list after the 1933 kidnapping by Kelly and his gang of Oklahoma millionaire Charles F. Urschel, for which a $200,000 ransom was paid (and which resulted in Kelly's eventual capture). This movie is loosely based on that incident.
The Florence 'Flo' Becker character was based on George Kelly's second wife Kathryn Thorne, aka Cleo Brooks.
American-International Pictures owners Samuel Z. Arkoff and James H. Nicholson wanted to make a gangster picture after years of making sci-fi pictures. This was the result.