Conceived and designed as a submersible commerce raider for the Royal Navy; at the time of her launching she was the largest submarine in the world.

For Britain, the idea of a submarine cruiser had been proposed as early as 1915, but the type was not put into practice until after the end of World War I in 1918.

Therefore, a certain amount of secrecy surrounded X1, the government even going to the lengths of taking a national newspaper to court over its pictures of the new submarine following her launch, all copies of the paper being seized.

[citation needed] The X1's 1-inch (25 mm) thick pressure hull was 19 feet 7.5 inches (6.0 m) in diameter amidships, and was divided into 10 watertight compartments.

[1] X1 carried four QF 5.2 inch Mk I guns[2] in twin unarmoured turrets, one forward and one aft of the conning tower.

Two auxiliary 1,200-horsepower (890 kW) MAN diesel engines taken from the German submarine U-126 were installed for battery-charging purposes.

[5] After X1 was commissioned in December 1925 and accepted in April 1926 she made a voyage to Gibraltar after which her main engine drive wheels were found to be damaged.