The 1940s brought us two highly influencial films which have remained the only two to-date to top Sight & Sound's poll as the #1 film of all-time: Citizen Kane (1941) and Bicycle Thieves (1948). In between we had the war years, and the birth of film noir. And it was the decade of Bogie & Bacall, Tracy & Hepburn, and Abbott & Costello, too.
Here's the 1940s films that have managed to elude me so far, but they're on my radar:
The Sea Hawk- 1940
Blossoms in the Dust- 1941
The 49th Parallel/The Invaders- 1941
Day of Wrath- 1943
The House on 92nd Street- 1945
That Lady in Ermine- 1948