In The Man of Bronze, Doc uses an older trimotor high-wing aircraft, similar to the Ford Trimotor Tin Goose, which fits in with the period, as a decoy to trigger an expected ambush. Described as an older aircraft that he planned on replacing with a new low-wing model, he used it as a sacrificial lamb to fool the criminal mastermind that was attempting to prevent Doc from discovering his legacy.
The make and model of Doc's trimotor airplanes are not identified in the books. Frequently they are outfitted as amphibians, capable of landing on land or water, by adding floats to the aircraft.
Of known aircraft of the era, the Junkers 52/3m is a close, but not ideal fit for Doc's trimotor airplane. It was a low-wing aircraft, introduced in 1931. It was also known to have been outfitted with floats for amphibious operations.