Jim,
I have a June 17, 1940 Ad from Life Magazine, which was focusing on Westinghouse Refrigerators, but in the background of the ad, depicts other appliances: Ironers, Wringer Washers, Water Heaters, Dishwashers (an Electric Sink is shown), Ranges, Irons, Roasters, Toasters and Vacuum Cleaners. This is the earliest Westinghouse ad that I have. So I think we can assume that Westinghouse had dishwashers, and/or Electric Sinks pre-WWII as well.
I also believe that I have seen, somewhere, pre-WWII dishwashers, marketed by Hotpoint.
Mike