The M2 started life in 1916 as K19 but due to major problems with the K class of submarines work was suspended after the keel was laid. A new class of submarines, based on the keel of the K class was designed. These were the M class, also called the “Monitor” class, of which the M2 was the second. These new submarines had as the main armament a 12-inch gun taken from an obsolete battleship as well as the normal torpedo tubes. She was launched in 1919 at the Vickers yard at Barrow. However the Washington Naval treaty of 1922 limited the size of guns on non capital ship to 8-inches so only three M class subs where built.
The M class submarines were very large for the time at 296 feet long (90m), they were designed to operate as part of the main fleet of battleships so needed to be fast. They displaced 1,600 tones on the surface and 1,950 when submerged. Two twelve -cylinder diesel engines powered them on the surface producing 2,400Hp, underwater electric motors produced 1,500Hp. The maximum speed on the surface was 15.5 knots, which proved to be slower than the newer battleships and the concept of fleet submarines was soon scrapped, with the River class of submarines being the last. British torpedoes at the time had a very limited range of at most 1000 meters and were notoriously inaccurate, if they did manage to hit the target most failed to explode. The 12-inch gun had a far greater range and accuracy and the 850Lb shell was a lot smaller than the 18-inch torpedo so more could be carried. However the gun could only be reloaded on the surface so was really a one-chance weapon.
On the 12th of November 1925 the M2’s sister ship the M1 was tragically sunk in a collision with the freighter Vidar. All hands on board were lost. Following public outcry the M2 and M3 were retired from active duty and assigned to an experimental flotilla. Here the M2 was converted to the first and only British submarine aircraft carrier, the conversion being completed in1928. The 12-inch gun was removed and replaced with a hanger. Inside a small Parnell Peto seaplane was stored. To launch her plane the M2 would start off submerged. The aircrew would enter the watertight hanger from inside the submarine and do all the initial flight checks including heating the engine with electric heaters, so as soon as the submarine surfaced the plane would be ready to launch. Once on the surface the large hanger door would be opened and the plane pushed out and attached to the steam catapult. The engine would be started and the wings folded out. Once this was done the M2 steamed into the wind and the plane was launched. The whole launch procedure could be completed in less than five minutes.
When the flight was finished the seaplane would land on the sea next to the submarine, a large boom crane would be swung overboard and the plane hoisted back aboard. The wings would be folded and she would be pushed back into the hanger. As soon as the hanger door was closed the submarine was ready to submerge. Initial trails were marred by a very unusual accident. Flying low over a beach the M2’s plane clipped a wooden beach hut. The collision caused the plane to crash and the beach hut to disintegrate, much to the annoyance of the occupier, Dr Lampblough. The naked Doctor promptly started to berate the pilot for dangerous flying, only realizing his state of undress when a large crowd gathered. The Doctor, who was unfortunately a very prominent local person, complained to the Admiralty and the captain of the M2 was removed from duty. The M2 was designed to operate in front of the main battle fleet and used her small plane as an aerial scout to locate the enemy battle fleet. Once located the main battle fleet would engage the enemy.