The original prototype for Super Smash Bros. was a four-player fighting game designed by Masahiro Sakurai and programmed by Satoru Iwata tentatively called "Dragon King: The Fighting Game". The game was designed specifically for the Nintendo 64's joystick to see how it could be used in a multiplayer environment. The signature concept of Nintendo characters fighting each other was not initially present, but would soon be implemented as the first idea Sakurai thought of to make his game stand out, as fighting games did not sell well and most of his original concepts felt better suited for arcade fighting games rather than home-console fighting games. A subsequent prototype featuring Mario, Donkey Kong, Samus, and Fox as playable characters was then made without sanction from Nintendo's higher-ups and they were not informed of its existence until Sakurai was sure the game was well-balanced. When Iwata initially presented the idea of the game to Shigeru Miyamoto to gain approval for the use of Nintendo characters, he turned it down. However, Iwata did not tell Sakurai this and convinced him to pitch the prototype to Miyamoto anyway, which later got his approval.

Little is known about Dragon King's gameplay or design, and all information known about the game comes from interviews and a handful of images shown when the game was in an alpha stage. No gameplay footage has been demonstrated, and a working prototype has not been made publicly available. Images of the game, however, show that much of the core gameplay was similar to Super Smash Bros., featuring damage percents, arenas with platforms, and combatants that do not strictly have to face one another. The tall, thin build of the unnamed fighters and their kicking and punching techniques also appear to be relatively similar to that of Captain Falcon. The most widely-known stage background featured in the screenshots is a photo taken by Sakurai of a Ryūō-chō neighborhood in Yamanashi, Japan, which is where HAL Laboratory's headquarters are based, where the game was being developed, and what the prototype was named after. The "Ryūō" in Ryūō-chō means "Dragon King".

According to an interview with Satoru Iwata, Masahiro Sakurai intended for the characters to use Final Smashes in this game. It was taken out due to hardware limitations. This concept was held off until Super Smash Bros. Brawl. Going in the debug menu you can find 3 sound files. 2 of them of Ness saying "PK" and "Starstorm!". The third is Captain Falcon saying "Come On!".

Luigi's voice is just a pitch-altered version of Mario's.

In the character select screen, the characters (excluding unlockable characters) are placed in the order of when they first appeared in their respective titles, starting with the oldest, Mario and Donkey Kong, and leading to the most recent, Pikachu.

While all other characters show their skeletons upon being hit with an electric attack, Samus's suitless outline becomes visible instead.

Captain Falcon is the only playable character that doesn't use his own head for his stock life icon. Instead, he uses a Falcon.

Captain Falcon's "Congratulations!" screen is the only one to use a pre-existing image from an original game, in this case, the first unlockable title screen from F-Zero X.

Luigi takes his appearance from Super Mario World, having purple overalls rather than dark blue. However, this is not true of his artwork.

When the Announcer introduces Metal Mario, his voice becomes slightly metallic. Although it may just be to place emphasis on Metal Mario's metal composition, the Announcer's voice sounds noticeably deeper and it does not change like that for any other character in Classic Mode.

Captain Falcon has the most usable costumes in the game, with six. Most characters have four or five. Yoshi also has six, but only four are usable outside of hacking, as two only appear in the single player mode.

It is impossible (at least without the use of a cheat device) to clear all eight digits in the points counter in 1P mode.

Charles Martinet is misspelled as Charles Martinee in the game's credits.

According to a page on the Japanese Super Smash Bros. site, Mewtwo was planned to be playable, but was cut; Bowser and King Dedede are also mentioned as having been cut, though no other information is available.

The Fighting Polygon Team is called Dummy Corps in the Japanese version.

For some reason, the announcer says Fox differently in the Japanese version than in the International version, even though Fox is a character with no name differences in any region.

Super Smash Bros. was developed with a low budget, had low publicity when it launched in Japan, and was even intended to be Japan-exclusive. However, the game's success prompted Nintendo to localize it for international audiences.

Captain Falcon and Ness are the only playable characters who do not have a stage based on their respective franchises.

In the background of the Mushroom Kingdom stage, Green Koopa Troopas can be seen walking back and forth across platforms. This contradicts every Mario game to date where Red Koopas turn when coming to an edge while Green Koopas will simply walk off the edge. This would later be corrected with the stage's reappearance in Super Smash Bros. Ultimate.

In an early version of the game, Peach's Castle lacked the sign, the bridge, and the sliding stone blocks.

One of Peach's Castle's hazards is a bumper, which is mostly used as an item. Coincidentally, this is the first stage to use an item as a hazard, and have a hazard outside of its own universe; Peach's Castle is from Mario and the Bumper is from Super Smash Bros.