For the scene where the P-40 Tomahawk crashes in the street, the effects guys used a real airplane and put it on a long ramp so that it would actually fly into the scene. No one knew how far it would travel before it came to a stop, so the cast and crew started a pool and placed bets on how far it would go. The day after they shot the scene, some of the crew walked into director Steven Spielberg's office and dumped a huge jar of money onto the desk in front of him. He'd won the pool.
1941 was directed by Steven Spielberg. The movie opens with a spoof of "Jaws", which Spielberg also directed. The woman who swims out and winds up on the submarine is the same actress who was eaten at the beginning of Jaws.
The U-boat in 1941 was a mock-up made just for the film. It floated, but did not work, so they had to tow it out to wherever they needed it for shooting. Also, to save money, they only built the front half of the sub, which is why you never see it past the conning tower in close-ups.
This movie was made before political correctness, and a number of racial slurs are made, just as they would have been in 1941. Amazingly, the film was released in Japan with Japanese subtitles. However, whenever someone used a derogatory slur, the subtitle simply translated it as "Nihon-jin" (Japanese Person).
The writers and producer originally wanted John Wayne to play the part of General Stillwell (a real general in WWII, played by Robert Stack). Wayne was excited about the part and asked for a copy of the script, unaware that it was a comedy. After he read it, he wrote a long letter to the producer of 1941, begging him not to make the film.
There are several instances where Bill Kelso is firing the guns of the P-40 (such as when Col. Maddox yells out, "Lemme hear yer guns!"). The special effects crew had many several challenges regarding this (including financial, since using the real thing would have been very expensive), so they wound up inventing a kind of mini-flame thrower to replace the plane's machine guns. When they were activated, gas was released in short bursts through the barrels of the guns, and a tiny electric spark (like the ones in gas stoves) ignited it. When activated, the guns appeared to be firing, but made very little noise and smoke. All the sound effects were added in post-production.
In the USO when the soldiers and sailors are squaring off on the dance floor, look carefully at the sailors. One of them is James Caan, who was filming another movie nearby and had some time to kill. When the fight starts, he throws the first punch.
There's a scene where Wild Bill (John Belushi) lands his plane on a highway in the desert and pulls up to a gas station to refuel. The gas station and the old woman that works there were also seen in Steven Spielberg's first movie, "Duel."
Though there are plenty of major (in 1979) stars in this movie, most of the advertising focused on John Belushi and Dan Aykroyd. Both actually had fairly small parts, and were never on screen together throughout the film. When Spielberg realized this during filming, he improvised a scene where Belushi and Aykroyd see each other and salute, just before Belushi's character "commandeers" the Japanese submarine. They appear to recognize each other, even though they never meet in the film.
After the boy turns on the lights at the amusement park, the Japanese sub fires a torpedo at it, thinking they're firing at Hollywood. Originally, they filmed a scene where the torpedo goes up onto land, goes between the boy's legs, and he rides around on top of it until it it hits a building and explodes. So the audience wouldn't think the boy has been killed, he's seen later running up to his sister and telling her that he's okay. Spielberg eventually cut the torpedo bit but left the part where the boy meets his sister. If you look, his clothes are disheveled and scorched from the torpedo explosion.