1990: Windows 3.0 - History of Desktop OSs
Windows 3.0 and especially the patched Windows 3.1 (1992) became the first Windows versions that would be called "operating system" by Microsoft and had a credible shot at mass market adoption as the PC gained much more market momentum with Intel's 386- and 486-processor generation.
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Still running on top of DOS, not representing the nature of an actual OS, and requiring command-line input to be launched, Windows 3.1 and Windows 3.11 for Workgroups (with networking support) are the first successful ancestors of the Windows operating system. Windows 3.x was installed from 14 3.5-inch floppy disks, supported 16 colors and was first to feature Microsoft's Program Manager, File Manager, and Print Manager.