The 40ft Armoured Target Boats were developed from the slightly smaller 37.5ft ATBs which had been designed by Scott-Paine and others at British Power Boat in 1932.
A couple of years later, in 1934, whilst bringing the first of the 64ft HSLs into service, it was realised by the Air Ministry that the condition of the aircraft had been advancing and that it was necessary to provide additional protection to improve the first type of Armoured Target Boats (the 37.5ft type). T.E. Shaw suggested to Scott-Paine that he should increase the length of the 37.5ft type to 40ft and fit twin rudders. In addition the Air Ministry prepared a new armour plating arrangement which gave seperate protection for thr crew and engines and coxswain. There was a further alteration to the forward bulkhead which resulted in it being changed to vertical instead of raked fore and aft to overcome the new conditions for bombing.
A long series of trials were carried out with the ballast with the 40ft type launch and eventually it was approved. The 40ft thus became the standard type Armoured Target Boat (ATB). The first batch of 15 craft of the 40ft type were ordered in 1935 with further batches being ordered in 1936, 1937 and 1938.
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