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. Windows NT 3.5x builds are known to install itself into the  folder instead of. This change was reverted in Windows NT 4.0. Support ended for NT 3.5 on the 31st December 2001, and Server 3.51's support ended on 30 September 2002.

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

It is the first version of Windows NT to upgrade to Windows 2000, as well as the second version of Windows NT to support upgrade paths to Windows NT 4.0 and the last version of Windows NT to be available in 3.5-inch floppy disks.

System requirements
According to Microsoft, the x86 version of Windows NT 3.5 and Windows NT 3.51 requires an 80386 processor, at least 16 MB of RAM, 95 MB of hard drive space, and a VGA or better display.

Hardware compatibility
Windows NT 3.5 does not support processor generations higher than the original Pentium due to a faulty processor check during setup. Windows NT will state the processor as incompatible and abort setup without additional patching of setup files. Windows NT 3.51 fixes the aforementioned issue.

Windows NT 3.5
Windows NT 3.5 (codenamed Daytona) 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 from IBM and others, 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 (exclusive to the Server SKU), 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.

Shell Technology Preview
The Shell Technology Preview, also known as NewShell, was a hybrid mixture of the NT foundation and the Windows 95 shell, acting as a preliminary version of Windows NT 4.0. Support for the Briefcase functionalities were only available if the user had installed Service Pack 5 on top of the existing Windows installation.

The reported version will be changed to 4.0 if installed. Due to this, many applications which are capable to run on Windows NT 3.51 refuse to install or launch due to requiring NT 4.0 with a Service Pack installed, e.g. Office 97.

Two builds were made for the Shell Technology Preview, both of which are listed below, and were respectively compiled/released in May and August 1995: