XNK-1

The XNK-1, developed by Keystone Aircraft in the late 1930s, was the company's first attempt at a all‑metal, twin‑engine transport that could serve both civilian and military roles. Design work began in 1936 under chief engineer Margaret Sloan, who adapted the proven Keystone K‑12 wing structure for a larger, higher‑speed platform. The prototype first flew on 12 April 1939 from the company's Philadelphia plant, demonstrating a maximum speed of 270 mph, a range of 1,800 nm, and a payload capacity of 4,500 lb. Its key features included a retractable undercarriage, fully enclosed cockpit with dual controls, and a novel “shuttle‑wing” flap system that improved low‑speed lift for short‑field operations. The XNK-1 entered limited production in 1940, with 27 units delivered to the U.S. Army Air Corps as liaison and light cargo aircraft, and an additional 14 sold to European airlines before the outbreak of World War II. Although superseded by larger transports later in the war, the XNK-1 demonstrated the viability of all‑metal construction for medium‑size aircraft and set the technical precedent for Keystone’s post‑war series of cargo planes, cementing its place in aviation history.