Betty Crocker and General Mills made cake mixes popular in the 1940's. There is print evidence of a Dromeday brand fruit cake mix in the New York Times on December 21st in 1932. Cake mix was a product of the industrial revolution but a specific inventor is not specified.
According to the Smithsonian Museum of American History, Charlotte Cramer Sachs developed boxed mixed for cakes, puddings, frostings, breads, muffins and popovers in 1948. The only item that needed to be added was water.