Because of the proliferation of heavily-fortified structures relied upon by all sides of World War 1 (1914-1918), large-caliber guns were the order of the way concerning that conflict. Siege warfare was alive and well and these massive creations were used to dislodge enemy forces and pulverize structures, preventing their reuse in the long term. Like the Germans, the British developed several railway gun types, large-caliber gun systems transported by the mature railway infrastructure offered in France, Belgium and elsewhere. One of these creations became the BL 12-inch railway howitzer weapon - and indirect-fire system that saw consistent use from 1916 onward. Eighty-one of these guns were manufactured for wartime service across three distinct, though related, marks - Mark I, Mark III and Mark V (Mark II and Mark V signified guns developed for British homeland defense in the war).
Design and manufacture of the units fell the Elswick Ordnance Company (EOC) (to eventually come under the Armstrong Whitworth brand label at a future date).
The 12-inch howitzers fired a 750lb High-Explosive (HE) shell of 305mm caliber at 1,175 to 1,468 feet per second depending on the model gun used. Effective range was out to 15,000 yards (again model dependent). As complete systems, they required considerable manpower and railway resources to be effective but their lethality on the battlefield was second-to-none. Artillery accounted for more deaths in the war, and more destruction of tanks, than any other weapon deployed.
The three designs varied slightly between each other: Mark I appeared in March of 1916 and sported a shorter barrel that its sister designs while mounting its recuperator / run-out cylinder above the barrel assembly. The Mark III followed by being given a heavier-duty breech section to contend with the inherently violent forces at play as well as to improve the weapon's basic balancing. In July of 1917, the Mark V arrived which relocated the recuperator to just under the barrel for a more natural arrangement and the breech was lightened. To offset this, an integral crew platform was added which rotating with the gun platform - offering 120-degree traversal. BL 12-inch railway howitzers were operated by members of the Royal Garrison Artillery for the duration of the war - which ended with the Armistice of November 1918. After the war many were give up for the scrap heap.
Their formal British Army designation was "Ordnance BL 12 inch Howitzer on Truck, Railway". A towed version also existed and is detailed in a separate entry on this site.