The BT-9 "Yale" Army Air Corps basic trainer evolved from North American Aviation's NA-16, the last prototype built in the General Aviation plant at Dundalk, Maryland. NJ-1. The BT-9, a two-seat monoplane with enclosed tandem cockpits, was of all-metal construction with removable fabric-covered side panels for easy maintenance. The engine and its accessories could be removed as a unit and changed in less than an hour.