I was around for this on the hardware side.
The TLDR is 64-bit NT was in the cards since early on since it was originally intended to be a CPU agnostic kernel that could run on anything and developers could just target one API (not ABI as they'd still have to compile to each architecture) and all of the hardware used for "real work" was 64-bit already or in Intel's case were going to be 64-bit by the end of the decade.
I don't know if NT 3.1 through 3.51 had 64-bit builds but 64-bit NT 4.0 definitely existed in-house. By that time they'd dropped MIPS and PowerPC support and NT on SPARC died in the crib. That was OK they thought because Itanium was going to save us all and make everything else obsolete. We know how all that went down.
But DEC went out of business and Itanium was late by years, so all of that work sat unreleased until the 64-bit versions of Windows Server and XP were publicly released in 2005. I've still got a DEC Alpha build of Windows NT 5.0 beta in a filing cabinet somewhere.
Out of all of this mess the only RISC chip from those days that's still standing and seeing active development is IBM POWER.
. Learning how to deploy, configure and maintain Windows NT 4.0 was... an enlightening experience to say the least, but I grew immediately fond of the OS. It was fun, and once you managed to set it up correctly (no Plug & Play support at all!), it seemed to be remarkably stable.
