Windows 3.x running in 386 Enhanced Mode had a very small multi-threaded preemptive kernel, which it used to handle its MS-DOS windows. So whilst each Windows program ran cooperatively within Windows and had no memory protection, Windows itself and each DOS window it opened were pre-emptively multitasked and had better memory protection. This wasn't very well documented, but it's the beginnings of Windows no longer running on top of DOS and instead taking over control of the machine.
Windows 3.1 also introduced "32 Bit Disk Access" which used a custom disk driver to bypass DOS and the BIOS and speed things up. Windows 3.11 (Windows for Workgroups) extended that to "32 Bit File Access", which bypassed DOS for file operations.
Windows 95 only used DOS as a bootstrapper. It would be completely incorrect to say that Windows 95 "ran on top of DOS", as once Windows 95 finished booting it had effectively pulled the rug out from DOS and was handling all I/O, memory operations, and so forth. It would be like saying that Linux runs on top of GRUB - GRUB is no longer in control of the machine, so it's just not true.
Not that I'm saying you were stating Windows 95 ran on top of DOS, you understand! I'm just putting this information here for educational reasons and expanding on your comment. ;-)
edit: I see Vice City has been removed following a DMCA request!
DPMI was pretty much an "Operating System API/ABI".
DOS basically acted as a bootloader. But all of those OSes had the very weird feature that they could switch back into a virtualised copy of their bootloader.
I do feel that Wikipedia understates the importance of Windows for Workgroups. Internally, it wasn't just Windows 3.1 with networking. It was a trial run for the fundamentals of the Windows 95 architecture.
When I worked at C_ we used to load Some solitaire game (Freecell) to verify that Windows98SE was in 32-bit mode before installing the network stack, and Chief Legal Officer, and from what I understand CLO was $4,000 a seat.
Load Driver, Reboot, Solitare, CLO. and then onward to disk optimizing, and then virus scanning... Two people did 89 machines, in 4 days. an entire floor... Food was delivered, and we slept for 4 hours, in the floor below, and on Friday, The head of Legal called us into his office... we showed him the checklist, as complete, and He laughed... the whole department was both amazed and happy.
He really called us to change his desktop into a scene from JAWS.
It was Windows 98SE that got a 32-bit disk driver upgrade, and FreeCell verified that it was installed.
Some windows games will run under the hx extender.