There was an assassination attempt on German ambassador Franz von Papen as depicted in this film. It occurred on February 24, 1942 and was carried out by the Soviet NKVD. However, in reality, the bomb malfunctioned and killed the would-be assassin - he did not get away by car as shown in this film.
After turning down The Maltese Falcon (1941) and All Through the Night (1942), as a condition for doing this film George Raft demanded his character be changed to an undercover FBI agent from the simple salesman he was in the 1937 source novel, where suddenly things begin happening to him that he doesn't understand and he gets pulled into espionage, à la North by Northwest (1959). After production wrapped on this film Raft bought out his contract with Warner Bros.
The three photographs of possible Roumanian secret agents which McNamara shows Joe were those of Warner Bros. contract players Paul Panzer, Glen Cavender and Stuart Holmes. Those three are not actually in the movie. The fourth photograph was that of Leo White, who is seen in an early scene whispering in someone's ear.
A contemporary article in The Hollywood Reporter noted Warner Bros. bought the film rights to Eric Ambler's novel as a vehicle for Errol Flynn.