In Alec Lumsden's book, "British Piston Aero-Engines...etc", on p 164, he says, "The Company (Napier Ltd) was also asked to build Sunbeam Arab engines. These were not satisfactory and Napier (i.e. Montague) was convinced he could design something better. In July 1916, he had begun design of a 12-cylinder, broad-arrow layout, PROBABLY INSPIRED BY A ROYAL AIRCRAFT FACTORY PROPOSAL (my emphasis).
Is there any evidence, anywhere that Lumsden's statement is true or was this just a myth that has floated around the literature for some time?