Xenix was Microsoft's OS before DOS/Windows, and it was UNIX-based.
Microsoft's forgotten UNIX operating system - Whatever happened to Xenix?
"Did you know that Microsoft made their own version of UNIX, all the way back as far as 1979, before MS-DOS and Windows was even a thing!? As usual, no story of computing folklore would be complete without a bit of controversy, and the story of Xenix is not short of a fair few crazy moments! Controversy aside, I bet you'll learn a good bit about a bit of history today, that could have changed the way we see and use computers a whole lot, if Xenix had won the day."
It implemented the tech to support multiple users and multitasking, the user manuals were written in LaTeX, and many of the Microsoft products we know have roots in Xenix. We can only dream of what could have been...