The atomic physicist Niels Bohr was extracted from neutral Sweden, after escaping from occupied Denmark, via this method in 1943.
The story goes (from Wikipedia):
Passengers on BOAC's Mosquitos were carried in an improvised cabin in the bomb bay. The flight almost ended in tragedy as Bohr did not don his oxygen equipment as instructed, and passed out at high altitude. He would have died had not the pilot, surmising from Bohr's lack of response to intercom communication that he had lost consciousness, descended to a lower altitude for the remainder of the flight. Bohr's comment was that he had slept like a baby for the entire flight.