[Image description: a screenshot of text paired with an image. The text reads “In 1939, in Kansas, Wheat mills owners realised that women were using their sacks to make into clothes for their children, the mills started using flowered fabrics for their sacks so the kids would have pretty clothes, and the label would wash out, a gesture of pure kindness”. The image attached is a sepia-toned photograph of a smiling woman wearing a green blouse and patterned skirt standing in front of a pile of wheat sacks that are using patterned fabrics. /ID end]
Actually, it started earlier than 1939, and it was smart marketing as much as anything else! Rural North American women used the cloth from feed and flour sacks to make towels, curtains, and other items as early as the 19th century. The companies making the feed and flour realized there was a market here, and they began adding patterns to the bags to encourage buying the same product every time. (For context, a 100-pound sack of chicken feed yields about 1 yard of fabric. 3 yards is the standard amount for a lot of adult dresses, and most people prefer that the parts of their garments match. So farm wives had an incentive to stick with, say, Gingham Girl products at least long enough to get enough gingham for a new dress.)
The making and wearing of feedsack clothing hit its peak in the 1930s and then began dropping off during WW2, when feedsacks switched to paper due to war rationing of cotton. But you can still find examples of feedsack patterns (special patterns made for use with feedsacks, sometimes printed right on the bags) floating around.
TL;DR, feedsack dresses and other clothes started WAY before 1939, mill owners knew about it long before that, and the pretty patterns were more about marketing than kindness. But turning feedsacks into clothing is still very cool of those long-ago rural folks to do.