Windows NT 3.1 build 175

Windows NT 3.1 build 1.175.1 is the earliest confirmed build of Windows NT 3.1. It was shown in Microsoft's promotional video from 1991, along with a pre-release build of Windows 3.1. 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 is also often referred to as the "July 1991 beta", despite the fact that no exact release date can be observed in the video or any other information related to 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 a pre-release build of Windows 3.1) to launch the Clock application, then minimizes both to reveal two instances of WINBez running in the background, along with another graphics demo app named simply "Demo" and two command prompt windows.

Date
Part 2 of the video shows the date “February 7, 1991” so the build was likely compiled around that date.