http://www.blogcdn.com/wow.joystiq.com/media/2009/03/hungry-man-dbreathchili425.jpg.
I'm kidding.
Maybe,
I dunno about a national dish, but I know about Murica's http://s3-media2.fl.yelpcdn.com/bphoto/xf7HbuALOJjGsBarlxUTKg/o.jpg!
*ahem*
I guess Hot dogs and hamburgers are considered a national food.
I think for the U.S. you'd have to skip the idea of a National Dish, and go with a State by State thing, or at least a regional view. Because outside of Apple Pie and Hot Dogs/Hamburgers, I can't think of anything that stands out as a national dish for the U.S., but I know there are plenty of regional/state dishes. For example, in Maryland it's Crab Cakes that we are well known for. And no, I don't eat it, I don't like Crabs or Lobsters, bleck. I can barely tolerate shrimp and that's only if they are breaded and fried.