Sonic the Fighters/Development

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Development process

While likely not intentional, Sonic the Fighters started its development during the creation of Fighting Vipers. During his spare time, an unknown AM2 developer added crude, but playable versions of Sonic and Tails to the Fighting Vipers codebase. Neither were finished and cannot be selected during normal play, but it was enough to attract AM2 head Yu Suzuki, who delivered the Sonic the Fighters concept to Hiroshi Kataoka^[1].

At first, Kataoka was worried that Sonic characters beating each other up wouldn't be approved by Yuji Naka, however Naka was actually very receptive to the concept. A decision was then made to continue working with the Fighting Vipers engine, modifying it to better represent the world of Sonic, and simplifying the gameplay. Sonic the Fighters remains very close to Fighting Vipers in terms of style, with one of several unused characters even resembling Honey from the aforementioned game.

Sonic the Fighters was the first game to render Sonic the Hedgehog as a 3D polygonal model in real time. Many teams at Sega struggled at the time to convert what was then a predominantly 2D character into the third dimension, and AM2 were initially no exception, however Sonic is unique for having been partially designed in 3D from the outset. A physical 3D model of Sonic, used by the artists of the original Sonic the Hedgehog to better understand how the character should animate, was reportedly given by Naka to the team for reference.

Early screenshots

First screenshots

Sonic the Fighters stems from Sega AM2's previous arcade fighting game, Fighting Vipers. Simple models for Sonic and Tails can be found within Fighting Vipers but are unplayable through normal means. When Sonic the Fighters was first announced, they finally had a time to shine.

It would be a while before Sega allowed people to play the game. For the first couple of months, characters would just stand around looking cool.
The background is basic, the floor of South Island is a curious checkerboard of blue and yellow, and the characters lack shadows. Sonic and Tails are noticeably off-model and have fewer polygons than the final game.
From a slightly later build, Sonic is looking more healthy now, though Tails hasn't changed.
A zoomed in version of the previous shot, also released by AM2.
AM2 also released some screenshots without any background at all.

In-game screenshots

A different build was used to demonstrate Sonic and Tails fighting each other. All of these screenshots are believed to have come straight from Sega, and depict a different South Island with a textured floor and barriers similar to Flying Carpet (with palm trees as the corners).

"Sonic vs. Sonic" in a distinctly Fighting Vipers-esque player select screen.
A screenshot from an emulator showing the full Fighting Vipers player select screen with Sonic and Tails.
Sometimes the characters have shadows.
Tails wins, performing a stance not seen in the final game.

Knuckles

Knuckles the Echidna was announced as a playable character, alongside the Casino Night stage. A set of screenshots were released showing him facing Sonic from different angles.

Casino Night is completely different, with no walls and a simplified background.

Amy

Next to be shown off was Amy Rose, and the Flying Carpet stage.

The bases for the candles are different.
A top-down shot shows the most obvious difference - the flying carpet doesn't fly. Instead it is presumably held in place by four stone pillars. The candle flames also do not constantly face the camera, so the illusion that they are not just 2D models is shattered in this shot.
Tails and Knuckles were later shown in this level. Tails is sporting his final design, but characters still lack shadows.

AOU Show 1996

A build of Sonic the Fighters was playable at AOU Show 1996 in February 1996. It is not entirely clear how much of the game was accessible at this point, though screenshots suggest most characters were playable and an AOU "Thank you for your playing" screen hidden within the final game's data reveals the game was approximately 50% complete. Aurora Icefield, Mushroom Hill and Death Egg's Eye make an appearance.

AM2 chose to make less of a fuss about the remaining four playable characters. Fang and Espio were announced first, followed by Bean and Bark. All were thought to be playable, though Bark is noticeably incomplete.

Hidden within the final ROM is this fullscreen image. This suggests the game was 50% complete by the AOU show.
In this prototype, player one always has a blue health bar, while player two has a yellow one. Perhaps a leftover from when Sonic and Tails were the only playable characters?
Also missing are the "barrier" indicators.
Aurora Icefield has a more opaque floor and a lighter sky, not to mention different ice walls.
Different winner text.
An over-the-shoulder shot, showing different K.O. text.
The mushrooms are all the same height and colour. In the final, this is varied slightly and there are bigger mushrooms in the background.
It looks as if a single player mode was also available. Knuckles takes on Sonic in stage 8, but it's South Island instead of Giant Wing. The stage has a smaller checkerboard floor with orange/yellow highlights instead of light green.
Getting to stage 8 in under 4 minutes is nigh-on impossible in the final game. This suggests not every stage was being shown.
Metal Sonic appears in stage 9, alongside the Death Egg's Eye stage.
Casino Night now has walls, but is still missing detail from the final game.
Bean's bombs are creating a lot of smoke. This effect is only seen for a small fraction of a second in the final game.
Fang takes on... Bark, whose design has yet to be finalised.
Bark is entirely white (as a polar bear perhaps should be), wears a slightly different hat, lacks a scarf and has different "hair".
There's little in this design that carries through to the final game. He was likely completely re-modelled.