I was fairly sure that NT was originally going to be a microkernel, but it ended up being really slow, so they had to push everything into a more monolithic architecture.
NT derived from Mica, DEC's microkernel OS project for their research RISC architecture PRISM. When DEC cancelled Mica along with PRISM — flirting briefly with MIPS before finishing Alpha — project lead Dave Cutler and some of his team took it to Microsoft. (The eventual settlement had MS support NT on Alpha until DEC collapsed from mismanagement.)