Just curious. It seems that for anything that’s an edible grain or plant starch someone has made “bread” with it. Are there any grains or plant starches that just don’t work as breads?
Cornbread doesn’t really seem much like “bread” to me. Closer to cake. But I don’t know if that’s a shortcoming of the corn. How do you define bread? My wife is a pastry chef and would probably know, but she’s asleep. I’ll ask her tomorrow.
AFIK (But I’m no expert) you can make breads from every starchy grain. However, for the bread to rise, to be airy, to be more then a flat solid cake, the dought has to be elastic. Wheat contains “gluten”, an elastic protein that causes the rubbery elastic substance of kneaded dough. IIRC, not all starchy grains have an equivalent of gluten.
Many X Breads are wheat breads with X in them. Potato bread, for example, is more wheat than potato.
The only real breads are made with wheat and added grain flour.
Doesn’t it depend on your definition of “bread”? Potato bread, if made with 100% potato flour, would resemble potatoes more than bread.
As stated, it depends what you call ‘bread.’ Rice makes poor bread (because of the low level of gluten?). Corn makes poor bread. As a result, bread was completely unknown in pre-Colombian America (I think, corrections welcome) and old Asia.
As Maastricht says, real bread (by which I mean yeast-leavened) can’t be made with low-gluten flours - which pretty much limits your options to wheat.
The gluten is what allows the dough to stretch to accomodate the bubbles formed by yeast during the rising phases, so no gluten means no rise. You can still use other forms of flour in bread-making, most recipes use some wheat flour to ensure there’s still an adequate amount of gluten.
Anyone who’s tried gluten-free bread can tell you what happens otherwise. 
If you’re using a looser definition of bread by referring to any food that involves a batter that is steamed, baked, fried or otherwise cooked, then there’s really no limitation whatsoever - for example, Asians make steamed buns using rice flour, Americans make cornbread, South Americans and Africans make dumplings from manioc, Scots make oatcakes, and so on and so forth… anything that can be pounded into flour and combined with water can be made into bread.
And of course, gluten-intolerant people have no choice but to eat disgusting dense crumbly stuff.