The novel upon which this film is based was published in 1943, and was considered a "sequel" to the same author's earlier novel, which had served as the basis for the 1932 Best Picture Oscar winner, Grand Hotel (1932). Production took place from late 1944 into early 1945, with the screenplay being continually revised to remain up-to-date on the fast-moving events of the final year of World War II into account.
The paper Koenig is reading in the cellar of the resistance is taken from President Roosevelt's radio address at a dinner of the Foreign Policy Association in New York City on October 21, 1944.
Andrea King was originally to have top billing, but when Faye Emerson married President Roosevelt's son, Elliott Roosevelt, the studio swapped their billing positions to take advantage of the situation. It is said that King took the change gracefully.
This and the other film directed that same year by Peter Godfrey, the Barbara Stanwyck romantic comedy Christmas in Connecticut (1945), were two of Warners' biggest box office hits of 1945.
The portrait of Himmler in Von Stetten's office is by Ernest Hamlin Baker, used as the cover of TIME magazine 24 April 1939.