So, I've been looking at the development of Windows Vista, which started out as Longhorn, and I've noticed that the earliest build of Longhorn is Milestone 3. I have not seen Milestone 1 or Milestone 2 builds, which leads me to this question: do the first 2 Milestones of Longhorn exist?

There are some M1 and M2 builds rumored to exist, however none of them are leaked as of now. There is some screenshots evidence of build 3663 existing, which would have been a Milestone 2 build. It's also possible that the first milestone was just a planning stage, which would mean that none Milestone 1 builds exist at all.

Yeah, I've looked in the Wiki page and I saw that there are the builds 3663 and 3670 of Longhorn, presumably M2. They probably exist, but they haven't been leaked yet. Thanks a lot for telling me. Cheers.

I doubt so, even on a very early stage of development, while planning what features will be implemented, there is still a little of development, especially if it is essential and was planned even before starting development of next version of software. This is especially true with drivers, where a very little needs to be planned, as it is just adding support for new hardware.

I think that Windows team had oviously made several Milestone 1 builds, but they could be just a very little changes or bug-fixes to operating system, or experimental features which they tested probably in branches.

Most of the rumored Milestone 1 build tags are taken from driver versions, so there's that, however these drivers also exist in Windows Server 2003, it's unknown at which point the Longhorn builds started to exist and everything before that was Windows Server 2003. All we can make right now are assumptions, because we have no evidence of any M1 builds at all

i'm probably assuming that the early build 3551 looks like XP rtm but with the windows .net server 2003 code in it. So basically it looks like XP with a few changes to it. So only build 3663 is the only Milestone 2 build we know.

If the M1 build tags were only taken from drivers, then it's likely that the ddk tools were next-gen, but relatively unlikely that the drivers themselves came from M1 builds of Longhorn. Remember, the underlying tools that build the OS are typically more important than the OS itself. Having a sane compiler is the only way to ensure that all your code is compiled with sanity in check.

We should really be trying to find the M1 DDK/SDK for longhorn

The M1 build tags are literally present in Server 2003, just with 5.2 instead of 6.0. They're Server 2003 numbers (probably not even from full builds) that somehow got added to a whole bunch of longhorn build lists under the faulty assumption of "every file version in Windows is another Windows build number".

Longhorn Milestone 1 and 2 builds definitely exist. The sidebar was in place quite early I understand - we're talking like Q1 2002 - though there was no Plex, which is no surprise.

They obviously do, we just don't have any.

Longhorn may have been a thing as early as Dec. 2001. In one Microsoft antitrust document, has this:

viewtopic.php?f=62&t=34527&p=398811

Well, that's certainly interesting. I really suspect that Neptune morphed into LongHorn - there's some anti-trust documents that outline the ideas that Neptune was going to have, and for the most part, those features made their way into LongHorn pre-Omega13. Exhibit 6451, for instance, is from 1998, describing stuff for Neptune. WinTone was a plan to make Windows as a Service, much like how Windows 10 is attempting to become.

LongHorn was likely built off of Neptune code, early on. M1 prototypes were probably closer to Neptune than LH. It was supposed to be the next major version of Windows, after all.

Although some of the guys on the UX/UI side of Longhorn came from the Neptune project I don't think Neptune exactly morphed into Longhorn. There was quite some time between them. Some ideas might have been used though. A major goal of the M1 concepts was (3rd party) service integration into the shell. Mind you, at this time (~ late 2001) Longhorn wasn't meant to be the next major Windows release - that was still planned to be Blackcomb.

Some time ago I've written up a post about the early development of Longhorn. This might be of interest to you: http://longhorn.ms/early-development. Adiitionally, if you take a closer look at the Avalon assemblies present in early build like 3683 you will find some references to the very early milestones. Some of the core assemblies contain debug messages. One specifically mentioned a M2: http://www.osbetaarchive.net/forums/top ... something/

Longhorn before 3673 IMO was technically XP with a possible extra feature or two. I'd imagine it would probably take a few builds to actually transform into something unique like the Longhorn we've all seen, plus the taskbar and other features inherited by Longhorn take time to build. Since each build deletes, add, or changes certain aspects in baby steps. (Excluding build 8102, which was a abnormal jump in progress compared to previous builds of Windows 8 beta), I'd imagine there would be nothing really too extravagant to see in M1 or M2.

Probably just a client srv03 with perhaps a WinXP(-like) UI with very minor changes I can't see any fascination with builds before 3663. While many say 3790.1232 was among the greatest leaks ever, if you take a good look at the OS it's quite boring except for the client, zero-day-activation, and "Longhorn" in the EULA.

you can't compare the reset build(s) with M1-M2
these to me are the most interesting, as you see what they started with and what were 1st integrated things

and who is to say that M1/2 had nothing interesting ?
do you have those builds ? - no you don't

check how win7 had good stuff early on
M1 had new taskbar
M2 had improved it and had new "style" explorer (better)

I can agree, but in a logical sense, there most likely wasn't much to look at before 3683... If there was, someone would've already managed to leak it at least some of it in the past 14 years...

not if they were internal only

Based on the development of Whistler to XP, I could assume that early Longhorn M1 and M2 builds might be minor changes from Windows XP and change the build number since then.