Long ago, NW flew IAD-ORD-NRT with its 747s, flight 3 I believe. Now that UA dominates at IAD, its code-share service with ANA is sufficient; pax also, of course, have the option of flying with UA to LAX, SFO or ORD and transfer to flights to the Washington DC area. If demand warrants, its possible that UA would add a 2nd flight on the route.
Also remember that many business travellers prefer flying into close-in DCA, and if they fly first to another US gateway, say ORD, and clear customs and immigrations, they can then catch a flight into conveninent DCA. This also happens frequently in the NYC area, where AA flies many pax from NRT to DFW and then DFW to LGA. If you take the total trip time (from door to door, not airport to airport), sometimes it comes out quicker to connect......you arrive in your destination city, have no potential customs delays, and are at least 45 minutes closer to downtown.