The U.S. Air Force only considered the first prototype to fall under the XB-43 designation. The second prototype, though functionally identical to the first, was designated YB-43, consistent with its status as a follow-on prototype under the Army's designation system for aircraft.
The U.S. Air Force only recognized the initial prototype as the XB-43 model. The second prototype, despite being functionally identical to the first, was designated YB-43 in accordance with the Army's naming convention for follow-on prototypes.