The early Windows versions weren’t really operating systems, as they required MS-DOS to be installed on the PC already. Windows was just a GUI, but it was still DOS under the hood.
Windows 95 was the first real Windows operating system.
Hard to believe it’s been 20 years since it came out!
Actually, no, Win95 and 98 still required DOS underneath, although once they were running they largely ignored it. Try starting a Win95 system without IO.SYS, MSDOS.SYS, CONFIG.SYS, and AUTOEXEC.BAT and you'll see what I mean.
The "NT" branch of Windows (which gave us NT4, Win2000, XP, and the rest) was the first true Windows OS family that was independent of MS-DOS.