1. Dungeons & Dragons co-creator Gary Gygax introduced the Magic Missile spell in the original game’s first supplement, Greyhawk (1975). “This is a conjured missile equivalent to a magic arrow, and it does full damage (2-7 points) to any creature it strikes.” After that sentence, the description tells how higher-level magic users shoot extra missiles.
2. Gary took the idea for Magic Missile from the 1963 movie The Raven. The movie ends with a wizard duel between Vincent Price and Boris Karloff. Karloff flings bolts of energy at Price, who brushes them aside with a flick of his hand.
3. The exchange that inspired Magic Missile also led to the Shield spell, so the original Player’s Handbook (1978) explains, “This shield will totally negate magic missile attacks.” This property remains in fifth-edition D&D.
4. The original description of Magic Missile led players to dispute whether casters needed to make a to-hit roll. J. Eric Holmes, the editor of the 1977 Basic Set, opted for yes. His rules explain that casters must roll the same missile attack as a longbow. TSR editor Tim Kask helped Gary plan Advanced Dungeons & Dragons. “The only thing that I won was that Magic Missile always hits for 1 to 3 points of damage,” Tim said. “It took me two-and-a-half weeks of arguing. I kept telling him that that’s the only thing the little guy gets and if it’s hit or miss, then he’s dead.”
Magic missiles always hit without allowing a saving throw, even though in the Dungeon Master’s Guide (1979) Gary stresses the importance of saves. Player characters “must always have a chance, no matter how small, a chance of somehow escaping what otherwise would be inevitable destruction.”
5. D&D’s fourth-edition designers seemed uncomfortable with a spell that always hit without a save, so the edition’s original version required an attack roll. When D&D fans griped that fourth veered too far from the game’s roots, the designers appealed to nostalgia by again making the missiles always hit. The 2010 rules update announces the change.
6. In fifth edition, wizards can add missiles by casting Magic Missile with a higher-level spell slot. In earlier editions, higher-level casters gain extra missiles for free. Back then, magic users started as weak characters who only launched one missile when they cast their day’s only 1st-level spell. But wizards steadily gained more spells, and higher-level spells, and even their first-level spells like Magic Missile gained strength. At higher levels, wizards boasted much more power than any other class. Gary Gygax felt comfortable with dominant, high-level wizards so long as they suffered through lower levels as feeble magic users. Today’s designers strive to match the power of every class at every level. Part of that balance comes from attaching a price to extra missiles.
7. In fifth edition, the missiles strike simultaneously. This means the strikes count as a single source of damage for things like resistance and that 3 magic missiles striking a character at 0 HP does not count as 3 failed death saves. A concentrating spellcaster hit by multiple missiles makes one Constitution save against a difficulty class set by the volley’s total damage. See 9 More Fifth-Edition D&D Rules Questions Answered by the Designers.
Update: In a newer answer to the same question, lead-designer Jeremy Crawford reversed the answer given at the convention Q&A. He now says the make separate concentration rolls for each missile. This makes Magic Missile an efficient way to break concentration.
8. Strictly by the fifth-edition rules, when you cast Magic Missile, you roll 1d4 and use the result to set the same damage for every missile. This stems from a rule on page 196 of the Player’s Handbook. “If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them.” The interpretation comes from lead-designer Jeremy Crawford. In practice, Jeremy allows players to roll separate damage for every missile, just like Gary did in 1975.