The Cayman was an amphibious passenger aircraft which Biggles used for several missions in Biggles in Borneo. The book describes the Cayman as a "big machine ... It looked like a flying-boat on wheels." It had two engines and was originally a twelve-seater passenger plane.
No such amphibian by the name of Cayman or with the description above exists which fits the timeframe of the Second World War. So why did W. E. Johns have to invent a fictitious aircraft here? After all, he uses real marine aircraft such as the Saro Cloud in the same book. Elsewhere he uses the Grumman Gosling. The Supermarine Sea Otter and Supermarine Walrus also existed as widely known amphibians in his time.
The answer to John's choice probably lies in him needing an aircraft with characteristics to fit the constraints of his plot in the book. He needed a large amphibian aircraft which did not have a very long range.
He needed a large aircraft because of the number of people who needed to be ferried to Lucky Strike in the first wave after the Cotabato rescue:
- Biggles himself
- Bertie, his co-pilot
- Angus Mackail
- Suba - he needed him to connect with the tribesmen back at Lucky Strike. They wouldn't have been pleased if he had been left behind
- Rex - he needed him to communicate with Suba
- General Barton - a high value prisoner to the Japanese
- the two women - obviously he would not want to leave them behind
- Jackson - who had rescued the two women
- two others - wounded prisoners. One was Fee Wong, who would play a large part in the plot subsequently.
This was already two pilots and 11 passengers. At the same time, the aircraft had to be an amphibian because it had to land at Lucky Strike. So real amphibians like the Saro Cloud and Gosling were ruled out because they were too small. Large enough marine aircraft like the Short Sunderland and Short Empire were ruled out because they were not amphibians.
That left something like the Catalina. The only problem is the Catalina has a range of more than 2500 miles, more than enough to make the round-trip to attack the convoy of barges in Malaya (Malaysia) from Lucky Strike, so Biggles wouldn't have fuel problems using a Catalina, and that would remove some dramatic tension from the plot.