The Fairmile B motor launch was a type of Motor Launch

Motor Launch

A Motor Launch is a small military vessel in British navy service. It was designed for harbour defence and submarine chasing or for armed high speed air-sea rescue....

 built by Fairmile Marine

Fairmile Marine

Fairmile Marine was a British boat building company founded in 1939 by the car manufacturer Noel Macklin.Macklin used the garage at his home at Cobham Fairmile in Surrey for manufacturing assembly which is why the boats he designed came to be called Fairmiles....

 during the Second World War for the Royal Navy

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 for coastal operations.
While the Type A had been designed entirely by Fairmile, the Type B design had come from Bill Holt of the Admiralty

Admiralty

The Admiralty was formerly the authority in the Kingdom of England, and later in the United Kingdom, responsible for the command of the Royal Navy...

 based on the lines of a destroyer hull and the detailed design and production was taken on by Fairmile.

Like all their designs it was based on total prefabrication so individual components could be contracted out to small factories for production and these arranged as kits that would be delivered to various boatyards for assembly and fitting out.

Altogether approximately 650 boats were built between 1940 and 1945.