Several songs from both Doom and Doom II: Hell on Earth are based on heavy metal songs from the 1980's and early 1990's.

The "NIGHTMARE!" difficulty didn't exist in the initial versions of the game. It was added in version 1.2 after players jokingly said that the "Ultra-violence" difficulty wasn't hard enough.

Doom has actually been made to run on graphing calculators, though it only runs for a short time before the batteries are completely drained.

The title of the fourth episode of Ultimate DOOM "Thy Flesh Consumed" as well as the level names are derived from the King James Bible.

In 1995, it was estimated that Doom was installed on to more computers than Microsoft's own Windows 95. This led Bill Gates to briefly consider buying id Software, so they could have the rights to the Doom name. This also led to the development of a Windows 95 port of the game to promote the operating system as a gaming platform.

According to John Carmack, the name Doom comes from a line in the movie 'The Color of Money'. In the movie, Vincent Lauria (played by Tom Cruse) shows up at a pool hall with a custom pool cue in a case. "What do you have in there?" someone asks. Vincent replies "Doom." with a cocky grin.

Cacodemon's general visual design is similar to that of the Beholder, a similarly one-eyed classic Dungeons & Dragons monster (with eye stalks instead of horns). In addition, the Cacodemon was created from a cropping of a creature that appears on the cover of Manual of the Planes, a Dungeons & Dragons expansion book. The creature itself is known as an Astral Dreadnought, and was created by Jeff Easley for that book.

The Cyberdemon lost his arm and leg during a fight against BJ Blazskovicz, the hero from the Wolfenstein series, during the events of Wolfenstein RPG.

The space marine makes many faces in the game depending on the player's status. One of these faces is a "shocked" face known colloquially amongst Doom fans as the "Ouch Face", which rarely makes an appearance due to a coding error in which the programmers used "(plyr->health - st_oldhealth > ST_MUCHPAIN)" instead of the correct expression "(st_oldhealth - plyr->health > ST_MUCHPAIN)". While this face was meant to show up if the player took 20 or more hit points worth of damage in a single tic, the error meant that it only displayed when the player "gained" 20 hit points while taking damage. An example of this can be seen when attempting to use the IDDQD cheat during the very end of E1M8: Phobos Anomaly.

A number of source ports fix this behaviour, allowing the Ouch face to display as originally intended.

In versions of Doom prior to 1.4, a room in the level "E1M4: Command Control" features a sector shaped like a swastika. According to John Romero, this is a reference to Wolfenstein 3D, an FPS previously developed by id where the protagonist fights Nazis.

In the map "E4M1: Hell Beneath" in the "The Ultimate Doom" release, there is a room that features the logo for the band "Nine Inch Nails".

Despite always going by the name "Doomguy" or just "The Marine" since he was never named in the games, the protagonist got a name in the novelization of the series as Flynn "Fly" Taggart.

In the Doom Press-Release beta, dated October, 1993, the BFG9000 had a very different firing sprite and animation. The animation consisted of multiple red and green plasma orbs firing. This was later changed to reduce strain on computers at the time.

In the PlayStation, 3DO, Atari Jaguar and Saturn versions of the game, the "I'm Too Young to Die" skill level is replaced with "I'm A Wimp".

Episode 1, Mission 4 (Command Control) changed between different versions of the game. In the initial release version (1.0), the map contained a Swastika-shaped arrangement of computers, in a reference to Wolfenstein 3D, an earlier id Software game. This was later changed in version 1.4, the "final" version.

The idea to use demons as enemies came from a Dungeons & Dragons campaign played by some of the developers, in which the game ended with demons spawning from a portal and overflowing the world.

The first level in Doom was actually created last. This is because John Romero wanted experience in level designing so he could make the first level his best.

The game began development as a tie-in game based on the Alien franchise. However, because id Software wanted total creative control, negotiations with 20th Century Fox fell through. Instead, they took influences from the movie, as well as the Evil Dead franchise for the Chainsaw and Shotgun weapons, and dropped the use of aliens in favor of a "demons from Hell on Mars" theme.

There are eight profanity-laden unused quit messages defined in the game's source code. One of these messages refers to a person named "Ron", who designer John Romero later identified in a 1998 interview as Ron Chaimowitz, CEO of GT Interactive. The first seven messages are under a comment saying "FinalDOOM?", but this was added by Bernd Kreimeier when he was cleaning the source for release to the public. These messages were never intended for the game Final Doom; they were "development mode only" messages written by John Romero.

In Episode 205 of "The Angry Video Game Nerd" in which the Nerd reviewed Doom, game designer Sandy Petersen claimed that the lack of music on the Atari Jaguar version was due to a monetary issue regarding the game's composer, stating in the composer's contract that he was supposed to be compensated the same amount of money for the Jaguar version as he was for the PC version, despite the Jaguar version selling less copies. Sandy Petersen ended by saying:


However, designer John Romero disputed this claim on Twitter shortly after the episode's release, claiming the lack of music was due to a lack of memory and CPU space available on the Jaguar cartridge.