Answer: OS / 2
In April 1987, IBM and Microsoft held a joint press conference to announce an ambitious new project, a next-generation OS / 2 operating system. The promise was a faster, better replacement for MS-DOS which would improve the entire computer experience.
The two companies had collaborated with the project in the last two years, but at the time of announcement it was still under heavy development ̵
Although OS / 2 had several clear advantages over MS-DOS and Windows, it was great at multi-tasking-it was finally discouraged in hardware repairs, poor adoption rates, and an increasingly Windows-centered PC market.
While IBM was trying to reinvent the OS / 2 several times over the next decade, then finally faded in obscurity. The latest major issue was 2001 and the support was discontinued in 2006. OS / 2 is based on a few small niche markets, such as banking and metro management systems, but is largely a footnote in the calculation history.
Image courtesy of IBM.