20 years ago this coming Saturday, Microsoft launched Windows NT, its first fully 32-bit OS. Technically, all of Microsoft's Windows operating systems since the launch of Windows 2000 are considered to be part of the Windows NT family, including Windows 8, since they all share the same 32-bit core that Windows NT introduced two decades ago.
Microsoft decided to mark the 20th birthday occasion a day early today with a quick post on a section of its Servers and Tools blog, with team member Mark Morowczynski noting, "It can almost drink legally!" He also links to an Amazon.com page for a book published in 2009 that goes over how Windows NT was first developed at Microsoft called, "Showstopper! The Breakneck Race to Create Windows NT and the Next Generation at Microsoft."
The first versions of Windows NT were made mostly for workstations and servers (appropriate for today's celebration of Sysadmin Day). The first version of the OS had the version number 3.1, so that it matched the version number of the consumer 16-bt Windows 3.1 OS. The last version under the Windows NT name was 4.0. Even though Microsoft doesn't technically sell the OS under the Windows NT brand anymore, it continues to keep track of its version numbers since all of the Windows operating systems are considered to be based on "NT Technology". Windows 8 and Windows Phone 8 are, for example, under the 6.2 Windows NT version number and the upcoming Windows 8.1 has the version number of 6.3.
You can check out Neowin's own "family tree" that shows the evolution of most of Microsoft's operating systems in a Trivia Tuesday post from October 2012.