In chapter IX of Biggles in Borneo, Biggles describes an unwelcome Japanese flying boat which had discovered his secret base at Lucky Strike: "Looks to me like a Mitsubishi Navy H-96 ... reconnaissance bomber ... three-engined job, carries a crew of six." Typically, when Johns describes an aircraft in such great detail, mentioning the manufacturer and model number, he is trying to refer to an actual aircraft type.
The only trouble is, the Mitsubishi H-96 did not exist. Mitsubishi never made any flying boats. So what made Johns think there was such an aircraft? The answer lies in what the Allied intelligence communities believed around the time Johns was writing.
In March 1942, the U.S. War Department published basic field manual FM30-38 "Military Intelligence: Identification of Japanese Aircraft". Pages 172-173 of this manual lists a "Mitsubishi 96 (Mitsu 96) Flying Boat", showing it as an aircraft of three engines and six crew, exactly as Biggles described.
Around the same time, in 1943, the National Aeronautics Council in the U.S. published the "Aeronautics Aircraft Spotters' Handbook". This book was widely distributed to the civilian population and also lists the same aircraft type. It is possible that Johns owned this Spotters' Handbook or an equivalent U.K. publication and was using it as a reference for Japanese aircraft types.
Interestingly, the same field manual does list the Kawanishi 90-2 three-engined flying boat which really did exist and which looked very similar to the H-96.