Windows NT 3.1 build 175

Windows NT 3.1 build 1.175.1 was shown in a promotional video from 1991 by Microsoft, along with a pre-release build of Windows 3.1. It is the earliest known build of Windows NT. Bill Gates himself demonstrated the new multitasking capabilities that were set to be introduced with Windows NT.

It was long believed this build was demonstrated at the Windows World 91 event, but clearly that is not the case in the video, nor is there any evidence that the video was shown during Windows World 91. This build was also often designated as the "July 1991 beta", despite the fact that no potential release date can be observed in the video or any other information about this build. Nothing suggests this build was ever released outside Microsoft.

Analysis of the video
The build number can be seen at the top of the video from 0:20 onwards. The full string is believed to say "NT 32-bit Windows [NTX86BLD] v1.175". The presence of this build string suggests this may be a checked build.

Bill Gates first uses the new File Manager (ported to NT from Windows 3.0) to launch the Clock application, then minimizes both and the Program Manager to reveal two instances of WINBez running in the background, along with another graphics demo app named simply "Demo" and two command prompt windows.