Neither Spencer Tracy nor Walter Brennan ever went to Africa during the making of this film. Stand-ins for both of them were used in the long shots during the safari sequences, and whenever Tracy or Brennan were shown on safari in close-up against African scenery, they were acting in front of a rear projection screen.
The date of Henry M. Stanley's famous quote, "Doctor Livingstone, I presume," occurred on Friday, October 27th, 1871.
Otto Brower and Osa Johnson followed Stanley's path through Kenya, Tanganyika and Uganda with their crew and returned with 100,000 feet of film for use in the safari sequences.
Not counting the archive footage in Land of Liberty (1939), this is the first movie that features two double Acting Oscar winners who had won their awards prior to this film's release: Spencer Tracy and Walter Brennan. Spencer Tracy had just won two consecutive Best Actor Oscars, while Walter Brennan had won two Best Supporting Actor Oscars for films released in 1936 and 1938, and would win another in1940.