NT was never a microkernel. It has an intenral microkernel-like design, though; it's divided into cleanly separated modules, and modules may only communicate using a special IPC mechanism. But like OS X, the classic pieces (file systems, graphics, device drivers) run in kernel mode.

Originally, graphics ran in user mode, but Windows 2000 moved it back into kernel mode for performance reasons. Vista got a new device driver model and a new graphics driver model (WDDM), which has a way for graphics to run partly in user mode. Vista and later also support user-mode device drivers.

Windows NT 3.1 was based on Microsoft OS/2 NT 3.0, it was originally Microsoft's version of OS/2 with the Windows 3>X GUI added to it instead of IBM's Presentation Manager.

The NT Kernel and OS was designed to run DOS, POSIX, OS/2 1.X CLI apps (But not GUI) and 16 bit Windows and 32 bit Windows apps. Later on OS/2 and POSIX support got dropped.

Microsoft OS/2 NT 3.0 was a rewrite of OS/2 for 32 bit systems, before that OS/2 was on 16 bit systems with 1.X and IBM and Microsoft fought over 2.0 standards, and Microsoft stopped support for OS/2 and focused on Windows instead. Then renamed their OS/2 to NT for New Technologies.

The Interesting thing about Windows NT was that Microsoft had plans to port it to different processors and eventually dropped those plans and stuck with X86 processors instead.

https://en.wikipedia.org/wiki/Windows_NT

It was designed as a modified microkernel.

> The NT Kernel and OS was designed to run DOS, POSIX, ...

What you're referring to were what the NT team called "personalities", or more formally "environment subsystems".

A subsystem was an API on top of the native NT API. Win32 was one such subsystem; users were exposed to the Win32 API, and weren't supposed to talk directly to the intentionally undocumented NT API (although some developers did reverse-engineer it eventually). POSIX and OS/2 compatibility was implemented as subsystems, but these were rarely used by anyone, and eventually removed in XP. DOS was not a subsystem, but rather its own thing.

Note that this has nothing to do with microkernels. In a microkernel architecture, things like file systems and device drivers live run as normal processes separate from the kernel. In NT, everything was in the kernel. These subsystems didn't act as part of a microkernel design.

Oh, and NT was not a rewrite of OS/2. It was written from scratch. The project was started back when Microsoft and IBM had a good relationship, and NT was originally planned to as OS/2 3.0.

> The Interesting thing about Windows NT was that Microsoft had plans to port it to different processors

The didn't plan it, they did it. I was looking after Windows NT on Alpha processors at one job. I never personally saw the NT on PowerPC or MIPS, though they did exist.

You got it wrong (your first sentence), WinNT3.1 is written from scratch by Dave Cutler and his group from 1989 to 1994. Originally there would have been a OS/2 compatible subsystem, and some ideas stem from OS/2 but NT has a lot of more similarities with VMS, the previous OS project of Mr Cutler at DEC.