Boeing built two XL-15s as proof of concept prototypes and then 10 YL-15s for evaluation as they competed for the U.S. Army Air Forces (which became the U.S. Air Force shortly thereafter) contract for a new L-bird, a liaison aircraft. Boeing ultimately lost the contract to Cessna which had developed the L-19 Bird Dog. The L-15, the last manned piston-powered airplane Boeing ever built, never went into production.