Doing It for the Art: An actual, functioning replica of an M3 Lee medium tank was built for filming. Part of the reason the movie went over budget.
Enforced Method Acting: Toshiro Mifune, who had been in the Imperial Japanese Military during World War II, grew disgusted with the lackadasical behavior of the actors (mostly Nisei) who were playing the submarine crew. So he took the expeident of actually acting like a Imperial Japanese Navy captain would with his crew, including shouting at them and administering at least one Bright Slap to the actors, all of this with Spielberg's permission. The actors shaped up, realizing that Mifune was not playing around.
Spielberg also said that for Raiders he really wanted to prove that he could do a movie in time and under budget (which he did) and this movie was one of the main reasons.
Hey, It's That Guy!: 1941 features both Blues Brothers, one year before The Blues Brothers movie was released... not to mention John Candy (who played Burton Mercer in that film) and a cameo from John Landis (who directed it). There is even at least one promotional picture of Dan Aykroyd and John Belushi on that military motorcycle, although they never actually share a scene in the movie.
Lenny and Squiggy appear as a pair of soldiers manning an anti-aircraft gun. (Their character names, Willy and Joe, are a Shout-Out to Bill Mauldin's popular WW2-era comic strip characters.)
A young Mickey Rourke, appearing in his first film, plays one of the tank crew under Aykroyd.
The original Zemeckis-Gale script was a Black Comedy entitled The Night the Japs Attacked.
A scene of Slim Pickens being threatened with what looks like a torture device but turns out to be a coat hanger was cut, but Spielberg liked the joke so much he swore that he'd try to put it in every one of his future movies until it stayed. He got his wish in his very next film, but just imagine if he hadn't and had held his promise during some of his later work...
Stanley Kubrick tried to talk Spielberg into making the film as a straight war drama. Later, Spielberg supposedly considered doing it as a musical.
Spielberg initially approached John Wayne to play Stilwell. Wayne, unaware that the film was to be a comedy, showed an interest... until he read the script, after which he refused in disgust at the film's "anti-American" tone.
The Ferris wheel subplot started as a vehicle for Jackie Gleason and Art Carney to essentially reprise their roles from The Honeymooners, to the point that the original script says the characters resemble a bus driver and a sewer worker. Gleason torpedoed the plan by mysteriously refusing to work with Carney again.
For a while there was going to be a scene where Wally is dancing along with a musical film behind the screen, and ends up falling through it, out of Joe E. Brown's mouth.