Steve Kanaly, who plays FBI Agent Sam Cowley in this film, played "Pretty Boy" Floyd in Dillinger (1973). Both films were written by John Milius (he also directed "Dillinger").
The ransom for the return for Thatcher Covington was set at $500,000. Since this was in 1933, the ransom, readjusted for inflation, would be just over $10.6 million in 2022.
In an interview published in the May-June 1982 issue of Film Comment magazine, this movie's writer, John Milius, expressed disappointment over the way his script was handled by the filmmakers. At one point, he referred to "that a--hole director, Dan Curtis." He then told the interviewer he could print whatever he wanted, adding "If he wants to see me, he knows where to find me."
When this film was released theatrically outside the USA, distributors considered that George "Machine Gun" Kelly was better known than FBI Agent Melvin Purvis. Promotion for the film emphasized Kelly, and the film was given several re-titles mentioning him, including "The Legend of Machine Gun Kelly".
Later in 1974 Harris Yulin (George "Machine Gun" Kelly) would portray J. Edgar Hoover in another TV movie, The F.B.I. Story: The FBI Versus Alvin Karpis, Public Enemy Number One.