Windows Longhorn

Windows Longhorn is the original never-released NT 6.0 version, that was known as such before the development reset: after it was known as Windows Vista.

Build 3551
Milestone 1 build, from mid-2001. Looks identical to Windows XP except for some changes in the winver.
 * Windows Whistler Theme
 * Windows Aquacolor Theme

Build 3663
Milestone 2 build, July 2002.

Build 3670
Milestone 2 build, August 2002.
 * Windows Longhorn Theme
 * BsOD Detector Support
 * Redesigned Notifications

Build 3683
Milestone 3 build, September 2002, this build features Desktop Window Manager and Plex interface theming, but the rest also looked like XP like M1/2.
 * Plex Theme
 * Lunar Theme

Build 3706
October 2002, features better working DWM.

Build 3718
November 2002.

Build 4000
Longhorn Under Construction, Revision 1, December 12, 2002.

Build 4008
Milestone 4 build, January 2003.
 * Red Screen of Death
 * Black Screen of Death
 * Antivirus, Crash, Bug and Virus Fixes
 * New Blue Screen of Death
 * Barney Error Detector Support
 * USB and Bluetooth Support
 * Emoji support
 * Windows XP Theme
 * Dark Mode
 * Darker Mode
 * Even Dark Mode
 * Darkest Mode

Build 4015
Milestone 5 build, April 2003.

Build 4029
May 2003.
 * Redesigned Taksbar Mannager
 * Keyboard Background
 * 3D and 2D Paint Support.
 * 3D Theme.

Build 4033
Milestone 6 build, June 2003.

Build 4039
August 2003.

Build 4042
Milestone 7 build, September 2003. First build to include Slate UI.
 * Including Slate UI.

Build 4051
October 2003, PDC demo build.

Build 4074
Milestone 8, April 2004. Shown at WinHEC, has feature complete file system/DWM/Aero, more or less, but may require workarounds for it to work in emulation software.

Build 4083
June 2004.

Build 4093
Milestone 9 (the only build), last one before reset in 2004.