Before id Software became huge following the release of renown first-person shooters, Wolfenstein 3D and Doom, they actually had some side-scrolling platforming under their belt... namely with Super Mario Bros. 3. At the time, the indie developers were producing a Super Mario demo for the NES.
According to IGN it wasn't until the 25th anniversary celebration of Commander Keen – id Software's less-than-popular side-scrolling platformer – that we find out about id Software's Super Mario Bros. 3 demo, which you can check out below.
John Romero posted up the video in lieu of the Commander Keen anniversary, explaining how at the time, before the team became renown as id Software, they completed a PC demo for Super Mario Bros. 3 on September 28th, 1990.
Unfortunately for id Software, the demo was rejected by Nintendo and the team went on to make Commander Keen later that year using the technology that they utilized for Super Marios. Between 1990 and 1991 the Commander Keen games helped build clout for id Software as they eventually transitioned into the first-person shooter genre with the 1992 release of Wolfenstein 3D.
In a way, it was probably best that Nintendo did reject id Software's attempt at making a Super Mario Bros. game, even though it looked very well-developed in the video above. Why was it for the best? Well, at the time Nintendo had a habit of scooping up developers as secondary parties to their endeavors: from Rare to Hudson, to Kemco and Natsume, the Big 'N' had a very loyal following and a very strict code on their quality seal of approval.
That's not to say that had Nintendo accepted id Software under their wing they would have stayed there, but it does make you wonder if Commander Keen had never been created – since they would have been working on Super Mario Bros. 3 – would it have led to the creation of Wolfenstein 3D and Doom? Maybe, or maybe not.
To think, though... without Doom the id Tech engine never would have gained ground and without the id Tech engine we never would have had Quake. Without Quake we wouldn't have Half-Life (which was based on Quake's Worldcraft technology) and without the success of Half-Life Valve wouldn't have created Steam. The butterfly effect on that decision Nintendo made back in 1990 could have changed the entire landscape of gaming.
It's fascinating to think about what would have been. In this case I think most gamers are happy where id Software is at the moment, and are equally thankful for their endeavors in pushing the first-person shooter genre forward by leaps and bounds with the release of Doom. Speaking of which, the reboot of Doom is actually due for release in the early half of 2016. So it's all coming full circle.
As for Commander Keen, the game series has been re-released recently on Steam as a complete pack for $4.99. Super Mario on the other hand has continued to move millions of units across various Nintendo devices for more than 30 years.