Microsoft co-founders Bill Gates and Paul Allen developed the Microsoft Disk Operating System, commonly known as MS-DOS, using existing software 'QDOS' from a Seattle company as a foundation.
Yet instead of selling it to IBM, Gates chose to retain the licensing for MS-DOS. This was a key point in in Microsoft’s evolution from a modest computer programming languages merchant to a leader in the computer software industry.

The MS-DOS release was timed perfectly with the growing popularity of the personal home computer, and was to be Microsoft’s greatest source of revenue, well into the introduction of their own GUI, Windows.