Thomas Broadbent and Sons Ltd helped with designing and actually built the X20 Midget Submarine. The X20 was a very well safeguarded secret and had been developed from its predecessors over the last couple of years.
Following its completion, it was disguised as a large Motor Boat, with tarpaulin and framework put all around the Midget Submarine. It was then transported under armed escort to Hillside Railway Sidings, and taken north to the River Clyde where she was laid down.
X20 would play a significant role in the D-Day Landings, acting as a key navigational tool for the incoming ships. This was done with the Submarine lying off the Normandy Coast from the 4th of June up until the Landings on the 6th. The Submarine had to remain undetected, along with its compatriot, X23.