Fred Zinnemann was almost taken off the movie because the studio thought he worked too slowly. He wasn't replaced because Spencer Tracy threatened to quit the movie if Zinnemann was fired.
There was no concentration camp at Westhofen at the time this film depicts, but there was one in the nearby town of Osthofen that was in existence from 1933 to 1934 when concentrations camps in Germany were re-organized and transferred from SA to SS control. The site in Ostofen is now a museum.
As a publicity stunt for the film, MGM employed Spencer Tracy's lookalike stand-in Ray Thomas to go to seven US cities, where there was a 'George Heisler man hunt'. The first person who spotted him in each city won a $500 war bond.