 For the same reason, the Thrush's single tailfin was replaced by twin tails on the Kingbird, and the main production model, the D-2 fitted a second horizontal stabilizer and elevator between these fins.Eastern Air Transport was to be the Kingbird's main operator, flying 14 of them for a few years.The United States Marine Corps also purchased an example, first designating it JC-1, then RC-1 and using it as an air ambulance.