The success of the British Q Planes spawned a brief cycle in airborne espionage pictures--at least until all British aircraft was impounded for actual combat use. The title Spies of the Air tells all: The central character is a test pilot who turns out to be in the employ of The Enemy. Since the film is adapted from Jeffrey Dell's stage play Official Secrets, much of the action takes place on the ground. The flight sequences blend stock footage and newly-shot aerial scenes with acceptable expertise. Spies of the Air was filmed in 1939, but not released until 1940--by which time its topicality had increased tenfold.

~ Hal Erickson, All Movie Guide