 The airstair was factory-fitted by Beechcraft during manufacture of the ultimate J50 model of the Twin Bonanza.The Queen Air development of the Twin Bonanza also received the Swearingen treatment, the Queen Air Excalibur having less extensive modifications, also involving fitting of IO-720s, replacing the more troublesome and lower-power geared Lycoming engines installed at manufacture, and the enclosed landing gear doors (the Queen Air was designed from the outset with an airstair).The United States Army had several of its U-8 versions of the Queen Air modified to this standard.