How nice it is to see that Johns had done his research well. The Persian Gulf geography mentioned in Three Weeks is accurate and the places actually exist, including Tumb Island.
With Ivan Nikitoff on board, Biggles had reached Jask and from there set a straight course for his planned stop at Lingeh, an Iranian port. This took him, as the book says, across the Strait of Ormuz (today Strait of Hormuz) and then across the "spur of Arabia named Oman" (today, the Musandam Peninsula). Over the Persian Gulf, and by his estimate, some 50 miles from Lingeh, he sighted Tumb Island. His aircraft developed engine trouble and he put down on the island for repairs and then decided to spend the night.
Tumb Island actually exists, and is today known as Tunb Island. There are two: the Greater and the Lesser Tunb Island. Given that Biggles' island appeared uninhabited, it is likely that he put down at the Lesser Tunb Island. This island is a barren rock some 25 miles from Lingeh and is about 2 square miles in area. Administered by Iran, it is also claimed by The United Arab Emirates. As in the story, the small rock is still uninhabited unless one counts the Iranian troops who have been deployed there to fortify it. A military runway and pier has been built on the otherwise undeveloped island. Given the unpromising description of real world accounts, it would appear difficult to even find a clump of stunted palms such as the one where Biggles set up camp.
Location of Tumb Island in relation to Biggles' route in Three Weeks.
Closer view of Tumb Island from a U.S. Army map of the 1940s. Lingeh is at the top left and the "spur of Arabia named Oman" is at the bottom right.