The XB 15 was the largest plane ever built in the United States until the Spruce Goose came along. The heavy bomber was so massive it had passageways in the wings and bunks for the crew. But big planes need big engines and no one made one big enough to give the XB any kind of speed for its maiden flight in 1937. The plane maxed out at 200 mph, and the U.S. Army Air Corps killed the project. The only XB ever built saw duty as a cargo plane in the Caribbean during World War II.