Windows NT 3.1 build 175

Windows NT 3.1 build 175 is the earliest showcased 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, where Bill Gates themselves would demonstrate the then-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 that event was held in May 1991, several months before this build was made (probably in early August). 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 reads "NT 32-bit Windows (NTX86BLD) v1.175". The same type of string is also displayed by build 196, which is a checked/debug build, confirming that the build in the video is a checked build as well. In Windows NT 3.5 build 782.1's source code, NTX86BLD is the name of a subdirectory in the  directory (which stores user settings of developers), which means NTX86BLD was the name of the user account which compiled this 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.

Part 2 of the video shows the date “February 7, 1991”. However, build 127, documented in the NT4 source code leak, was being tested on or near 1 May 1991, so this build was almost certainly compiled later than that.