Windows NT was allegedly partly developed on OS/2. Many MSers loved OS/2 at the time -- they had co-developed it, after all. But there was more to it than that.
Windows NT was partly based on OS/2. There were 3 branches of the OS/2 codebase:
[a] OS/2 1.x – at IBM’s insistence, for the 80286. The mistake that doomed OS/2 and IBM’s presence in the PC industry, the industry it had created.
[b] OS/2 2.x – IBM went it alone with the 80386-specific version.
[c] OS/2 3.x – Portable OS/2, planned to be ported to multiple different CPUs.
After the “divorce”, MS inherited Portable OS/2. It was a skeleton and a plan. Dave Cutler was hired from DEC, which refused to allow him to pursue his PRISM project for a modern CPU and successor to VMS. Cutler got the Portable OS/2 project to complete. He did, fleshing it out with concepts and plans derived from his experience with VMS and plans for PRISM.