Most sources assert that the Bell XH-15 was a four-seat helicopter, but judge for yourself. The Bell 54 or XR-15 design lost out to the Firestone XR-14. which never materialised, and three XH-15s were eventually constructed. Relatively powerful with an O-470 engine of 285 hp, the type had a reported ceiling of 20,000 ft and was used for high-altitude trials by the USAF. This photo of the first prototype was presumably taken at KIAG. Photo from: Bell