Don Siegel is a solid director who knows what to do with certain projects. He made Riot in Cell Block 11 for $300,000. My favorite of his films, Invasion of the Body Snatchers, went for a bit over $400k. Sam Peckinpah was involved in those films and others, and he considered Siegel a mentor. Siegel directed a movie starring Fabian. He directed one of Elvis' more highly-regarded films, the western Flaming Star (that came just before Hell Is for Heroes). He directed five movies with Clint Eastwood, who is the clearest example of a Siegel-influenced director.
He wasn't the kind of director who knew what to do with every project, though. His last movie, Jinxed!, is known for the rancor on set between Siegel, Bette Midler, and Ken Wahl. Siegel had a heart attack and never directed again. (And really, there isn't anything you can do with a Fabian movie.)
Hell If for Heroes is one of his better films. Star Steve McQueen was, well, Star Steve McQueen, but the movie got done without anyone killing each other, although McQueen and Siegel might have been tempted. (Add Bobby Darin to the list of pop stars Siegel directed, and also actors who quarreled with McQueen. Overhearing someone saying that McQueen was his own worst enemy, Darin famously replied, "Not while I'm still alive".) Once again, Siegel was working with an insufficient budget ... the ambiguous ending came mostly because they ran out of money.
The war action is intense, and Siegel doesn't make the common mistake of presenting his actors in such a way that their heroism overcomes any anti-war sentiments in the film. McQueen is a bit nuts, there's no real outcome to the soldiers' mission, some people die, war goes on. It mostly works ... Hell Is for Heroes is Don Siegel at near his best.
There is one particularly odd big of casting. Bob Newhart had become all the rage for his comedy albums, and the studio wanted him in this movie, which would be his first. His presence is rather like Ricky Nelson's in Rio Bravo ... give the kid a couple of songs and keep him out of the way otherwise. Newhart's character quickly ends up with a phone, and he spends much of his screen time concocting fake telephone conversations to fool the Nazis. Since his comedy act was almost entirely Newhart pretending to talk on a phone, these scenes turn out to be hilarious for the wrong reasons ... you can't help but laugh at the idea of Bob Newhart in a war film, talking on a fake phone.
Hell Is for Heroes isn't great, but it's a cut above other similar films, which is about the best you get from Don Siegel: a cut above.