Windows NT 3.5x

Windows NT 3.5x is a family of NT-based operating systems released by Microsoft. These versions of Windows are the last of the NT series with the Windows 3.1-style user interface, as Windows NT 4.0 would introduce the Windows 95-style shell. These versions succeeded Windows NT 3.1.

Notably, an early version of the Windows 95-style shell is available for Windows NT 3.51 via the NewShell update. NewShell replaces Program Manager and adds a new desktop, taskbar, and user interface graphics.

On 15 May 2020, the source code for Windows NT 3.5 build 782.1 was leaked.

Windows NT 3.5
Windows NT 3.5 was released on 21 September 1994. It is the first version to feature the edition names of 'Workstation' (previously called simply "Windows NT") and 'Server' (previously called "Advanced Server"). It cannot be installed on a computer with a processor newer than the original Pentium without modifying setup files. A very rare PowerPC version is also known to exist, but it was downplayed in favor of advertising it on 3.51. (note that this edition and other Windows NT versions supporting PPC only support PReP PowerPC machines, not Macintosh machines that use OpenFirmware)

Windows NT 3.51
Windows NT 3.51, released on 30 May 1995, is the first release of Windows NT to introduce PCMCIA support, NTFS file compression, replaceable GINA, 3D acceleration in OpenGL, Windows 95-styled common controls, as well as new versions of Sound Recorder, Media Player and Windows Help applications. It is the last Windows version to contain the Write word processor as well as the Program Manager that was introduced back in Windows 3.0.

Shell Technology Preview (NewShell)
The Shell Technology Preview, also known as NewShell, was a hybrid mixture of Windows NT 3.51 and Windows 95. This was a very early prototype version of Windows NT 4.0. If chosen to install it on the RTM build of NT 3.51, there isn't any Briefcase support, but if installed on Service Pack 5, you get Briefcase support. Two builds were made for the Windows NewShell, both of which are below, and were compiled/released in May and August 1995, respectively.