If you make the OS bullet-proof you don't need anti-virus protection in the first place...........
That's called OS X
Yea.
Step one. Remove 99.999% of the features of today's modern OSs.
Step two. Limit device support to a small number of devices that con be supported with simple drivers developed in house.
Step three. Do not allow any third party drivers to be loaded.
Step four. Only allow verified and tested third party software and only give that software a very limited API to interface to the OS with.
You've now got a decent starting poing for making a solid and secure OS.
Add in some rigid design procedures and outside test and verification, and if you're reasonably efficient, you'll only spend a couple thousand dollars per line of code for something arguably solid and secure.
For more information se the D.O. 178 standards for software design used in avaition system.
Ain't no such thing and never will be.
Simply put, there is no "bullet-proof" OS. If you think you have a bullet-proof OS then I invite you to keep on running it without any AV. All software is manmade and is therefore flawed. It is impossible to account for everything that could be attacked within a system.