The Bell L-39 was a one-of-a-kind research aircraft built by modifying a production P-63 Kingcobra with swept-back outer wing panels. Flown in 1946, it allowed Bell Aircraft engineers to study the low-speed stall and handling characteristics of swept wings using a proven, stable airframe rather than risking an all-new jet design. Data gathered from the L-39 program contributed to the aerodynamic understanding behind America's first generation of swept-wing jet fighters, including the North American F-86 Sabre. Project: 218475