To be precise, Windows 3.x did multitask, at least on a 386 - however, only on the much more primitive, cooperative level (which the application needed to allow and some just didn't). Amiga OS offered preemptive multitasking from the beginning - a feature which required 10 years to reach the mainstream Windows platform (Windows 95).
It did on the 286 as well.
I think the preemptive multitasking feature of AmigaOS is mostly overrated. Without memory protection, preemptive multitasking doesn't result in a significantly better end user experience. It's all fun a games until a program goes off into the weeds and then the result is the same with either setup, i.e. the user gets to hit the reset button.
Love the Amiga to death but back in the day when I first got my hands on a 386DX/25 w/ 8MiB of RAM and 1MiB trident VGA board, I didn't miss the Amiga one bit. I'd go so far as to say that Windows 3.1 was more sophisticated than classic AmigaOS ever was. It certainly was more stable.