During World War II, after crash-landing or being forced down, approximately 40 B-17s were captured and refurbished by the Luftwaffe with about a dozen put back into the air. Given German markings, the captured B-17s were used to determine the airplane's vulnerabilities and to train German interceptor pilots in tactics. Others, with the cover designation Dornier Do 200, were used as long range transports by the special duties unit Kampfgeschwader 200, carrying out agent drops and supplying secret airstrips in the Middle East and North Africa. They were chosen for these missions as more suitable for the role than available German aircraft and not in an attempt to deceive the Allies, being operated in full Luftwaffe markings. One of the B-17s of KG200, bearing Luftwaffe markings A3+FB, was interned by Spain when it landed at Valencia airport, 27 June 1944, and remained there for the rest of the war. Some B-17s kept their Allied markings and were used in attempts to infiltrate B-17 formations and report on their position and altitude. The practice was initially successful, but the Army Air Force combat aircrews quickly developed and established standard procedures to first warn off, and then fire upon any "stranger" trying to join a group's formation
