These tunnels were constructed as an extension to the existing factory in 1941. The building above ground was considered imprudent due to the vulnerability of the Medway estuary to air attacks. The two tunnels that housed the factory workshop area were both 100m long and were linked by four traversing tunnels of slightly shorter length.

The Shorts Brothers also constructed a large system of air raid shelter tunnels, which comprised two main tunnels some 300m long, traversed by 14 smaller connecting tunnels. The shelter tunnel complex was connected to the factory complex via a single tunnel approximately 400m long.