http://en.wikipedia.org/wiki/Boeing_B-17_Flying_Fortress
Luftwaffe captured B-17s
During World
War II, after crash-landing or being forced down, approximately 40 B-17s were
captured and refurbished, with about a dozen put back into the air. Given
German markings on their wings and fuselages and Nazi swastika
tail fin-flashes, the captured B-17s were used to determine the B-17's
vulnerabilities and to train German interceptor pilots in attack tactics.
Others, with the cover designations Dornier Do 200 and Do 288, were used as
long-range transports by the Kampfgeschwader 200 special duties unit,
carrying out agent drops and supplying secret airstrips in the Middle East and
North Africa. They were chosen specifically for these missions as being more
suitable for this role than other available German aircraft; they never
attempted to deceive the Allies and always wore full Luftwaffe markings.
One B-17 of KG200, bearing the Luftwaffe markings A3+FB, was
interned by Spain when it landed at Valencia
airfield, 27 June 1944, remaining there for the rest of the war. Some B-17s kept their Allied markings and were
used by the Luftwaffe in attempts to infiltrate B-17 bombing formations
and report on their positions and altitudes. The practice was initially
successful, but 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.