Taken from http://www.microsoft.com/billgates/speeches/industry&tech/uexpo.asp
If we go way back in time, Microsoft was actually the first one to go to AT&T and beg to get a nice high-volume commercial license for Unix. And for many, many years we were the highest volume licensee, not only for our own Xenix products, but Siemens with theirs, Santa Cruz with theirs, and dozens and dozens of sub-licensees.
I have to admit, it was fairly difficult to work with AT&T back then. They simply didn’t understand what they had. They didn’t understand how to manage the asset, either in terms of promoting it properly or in terms of making sure that there wasn’t fragmentation in how different implementations were put together. And so that vacuum in leadership created a bit of a dilemma for everybody who was involved in Unix.
Well, Microsoft stepped back and looked at that situation and said that the best thing for us might be to start from scratch: build a new system, focus on having a lot of the great things about Unix, a lot of the great things about Windows, and also being a file-sharing server that would have the same kind of performance that, up until that point, had been unique to Novell’s Netware.
And through Windows NT, you can see it throughout the design. In a weak sense, it is a form of Unix. There are so many of the design decisions that have been influenced by that environment. And that’s no accident. I mean, we knew that Unix operability would be very important and we knew that the largest body of programmers that we’d want to draw on in building Windows NT applications would certainly come from the Unix base.