In attempting to keep step with the German Army and its successes during the early stages of World War 2, British authorities saw the need to replicate a heavy-class smoke-generating mortar for its Royal Engineering corps. The requirement then evolved to produce a complete heavy-class field mortar that could also make us of lobbed explosive munitions against enemy positions producing the "Ordnance, ML 4.2-inch Mortar". The 4.2" model was added to the existing British Army stock of 2" and 3" infantry mortars already in service.
The 4.2" mortar weapon was of 106.7mm caliber and its design largely following the form an dfunction laid down by the original British Stokes mortar of 1915. Its development stemmed from the Armaments Research and Development Establishment (ARDE) with manufacture from the Royal Ordnance Factories (RAF) beginning in late-1941. The 4.2" was formally adopted into frontline service during 1942 and around 3,800 units were eventually produced.
Early forms of the mortar were fielded with a tripod support assembly and baseplate. On the whole the weapon looked the part of a traditional battlefield mortar with the complete system utilizing a smoothbore launch tube, supporting legs, ground recoil plate, and sighting/ranging controls. A crew of six was the typical fielding with the weapon due to its size and weight and the weapon hauled into action behind a small tracked carrier or JEEP-style vehicle.
The weapon held an elevation span of +45 to +80 degrees for suitable indirect fire upon enemy positions - primarily utilizing a standard High-Explosive (HE) shell (there also existed a smoke-generating projectile and practice rounds). Traversal was +10 degrees to either side form center until the whole system had to be turned into the direction of intended firing. A well-trained and experienced crew could reach up to 20 rounds of sustained fire for up to a minute while typical firing was in the 10 rounds per minute range. Maximum engagement ranges reached between 3,300 yards and 4,400 yards giving the 4.2" mortar a good command of the battlefield. The standard HE projectile weighed some 20lb. However, British defense industry - stretched to its limits by 1941-1942 - made due with limited-capability "cast" bomb bodies over the intended forged types - this was reflected in the broad engagement ranges listed above.
The weapon saw its baptism of fire in the North Africa campaign during El Alamein where the mortars were issued within British "chemical warfare companies" until dedicated heavy mortar companies were arranged during 1943. The weapon went on to see widespread service in the European and Pacific campaigns (including service with Australian and Canadian forces) in time with good results. By the end of the war, the weapon discarded its tripod support assembly and baseplate in favor of the Mk 1 Mobile Baseplate Trailer - a wheeled platform that aided quick transport and firing for crews, requiring just two dedicated personnel to operate minimally.
The competent 4.2" mortar design endured into the post-war years where it was still in play during the Korean War (1950-1953) where its indirect fire capabilities played well in the rugged, mountainous terrain of Korea. The weapon line also found foreign service with the militaries of Ethiopia, Laos, Malaysia, Nepal, and Turkey before its history had been fully written.