Wheat has traditionally been seen as the finest type for breadmaking. At least in the western world, which is where this answer will be based. What you referred to as "bleached" and "unhealthy" is basically interpretation of what wheat bread tradionally has looked like. White, soft and generally sweeter than bread made from darker types of grain, and thus more desireable.
Already in antiquity we have the greek lawmaker Solon declaring that bread made from wheat was only allowed during feastdays. (I can't locate my copy of "History of Food", but the Wikipedia article mentions it as a primary source as well. It's listed by author's name, Toussaint-Samat). Additionally bread made from wheat was seen as more desireable and nourishing (according to Diphilus, source: "Food in History" by Reay Tannahill p. 91. The same Wikipedia article actually mentions this as well, however here I managed to find my own copy). This was probably partly (or maybe mostly) due to the fact that low quality flour was abundant, containing anything from animal feces, chalk, clay, sawdust etc etc. Now are these types of wheaten bread alike to our modern day toastbread and similar breadstuffs? Probably not, but the tendency is there.
One of the aspects of the industrial revolution in the mid 1800 hundreds was the increased accessability of foodstuffs to the middle class, types of which were traditionally seen as either expensive, or luxuries associated with either the nobility or certain feastdays. (We still celebrate the danish holiday "Store Bededag" or "Great Prayer Day" by eating a certain type of wheaten bread.) Greater accessability to a growing consumer base meant higher demand, which is assumed to have caused the whiter breadforms to have generally eclisped the more traditional darker bread types. (I say assumed because actually studying long term supply/demand impact over a period of some 200 years is difficult at best).
It's worth noting however, that certain parts of the western world still hold to darker breadtypes. Scandanavia and the northern parts of Germany consume a lot of a type of bread made from Rye, typically a sour-dough based bread containing various seeds and maybe a bit of beer. This type of bread is a lot more nutrious, and quite tasty with the right accesories.
Hope you enjoyed, and I apologize for the shameless Wikipedia linking, but couldn't find any .pdfs of the books in question.
Our man on bread, /u/agentdcf, has written quite a lot on the history of bread - check these out
At the time of writing none of the posts here deal with the historic removal of the bran or germ from wheat (this is what makes "wheat flour" "white flour", not bleaching). The germ is where all the vitamins are and the bran is where all the fiber is, and this is typically what people lament when they say white flour is unhealthy. Does anyone know when it became common (or was first done) to remove bran and germ from wheat to make flour?