Microsoft used an ARM build of Windows CE for Windows Phone 7, 7.5, 7.8, before they jumped to running NT on their ARM phones. For sure it exists.
I can't figure out if you can actually get it though. They have "Windows Embedded Compact 7", but from the documentation, it sounds like it's only x86 and amd64. But maybe there's some way to get the ARM version... maybe an MSDN license or by being some kind of business partner with Microsoft?
On second thought, this link: http://www.microsoft.com/windowsembedded/en-us/windows-embedded-compact-7.aspx suggests that it does work for ARM.
Bonus: Apparently Windows 10 will run on the Raspberry Pi 2: http://arstechnica.com/information-technology/2015/02/windows-on-arm-lives-on-even-as-it-dies/ The only question is if they'll keep requiring every non-app .exe to be signed by Microsoft like they did with the surface RT, preventing normal win32 or .NET programs from being possible to develop for ARM on NT. If they don't give us that crazy restriction, this could be pretty neat.