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 ̵1; the press conference marked the first of many promises before delivery in the development of OS / 2. Each edition of OS / 2 became gradually more delayed (the release released in December that year did not even have a GUI) and in the end of 1990, the relationship between Microsoft and IBM was unraveled. Windows 3.0 enjoyed extensive success, with MS-DOS directly coupled with it, while OS / 2 suffered from stationary sales and poor driver support.
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.