I think it's important to note (even in a tl;dr) that for a time OS/2 was a joint venture between IBM and Microsoft, and that MS sabotaged that relationship while secretly working on WinNT.
On a related note, "Showstopper!: The Breakneck Race to Create Windows NT and the Next Generation at Microsoft" is a surprisingly entertaining story, and reads more like a novel than a documentary/memoir.
As a result of a feud between the two companies over how to position OS/2 relative to Microsoft's new Windows 3.1 operating environment, the two companies severed the relationship in 1992 and OS/2 development fell to IBM exclusively.
Windows 3.0 was eventually so successful that Microsoft decided to change the primary application programming interface for the still unreleased NT OS/2 (as it was then known) from an extended OS/2 API to an extended Windows API. This decision caused tension between Microsoft and IBM and the collaboration ultimately fell apart.
On a related note, "Showstopper!: The Breakneck Race to Create Windows NT and the Next Generation at Microsoft" is a surprisingly entertaining story, and reads more like a novel than a documentary/memoir.