October 31, 1988 – IBM officially shipped OS/2 version 1.1, which was where the graphical environment, called “Presentation Manager,” was first introduced (1.0 had no GUI). Co-developed by Microsoft and IBM, the OS/2 Presentation Manager bore many graphical resemblances to Microsoft's Windows. Presentation Manager's graphical API was a bit more advanced (having more powerful drawing API's) but was never as successful. Shortly after launch, the relationship between Microsoft and IBM soured. IBM went on to develop OS/2 on its own.
Fun Fact: Steve Ballmer stated that, if IBM shipped OS/2 2.0 before the end of 1991 he would eat a floppy disk. IBM shipped a “Limited Availability” release of OS/2 2.0 in November of 1991. Steve never ate that floppy.