The Imperial German Army enlisted several "trench mortars" during the fighting of World War 1 (1914-1918). This included the 7.58cm and 17cm minenwerfers (literally "mine launchers") but none were larger than the 25cm schwerer Minenwerfer heavy trench mortar. The system was designed during the span of 1907 to 1909 and eventually adopted in 1910. Production spanned into 1918 which increased the initial war volume from just 44 units to an impressive 1,234 by the end of the war.
Like other world military powers, the Germans observed the unfolding events of the Russo-Japanese War (1904-1905) with interest and took observations to heart when evolving their own modern doctrines. The war marked the first true conflict of the 20th century. The events of the Siege of Port Arthur (July 30th, 1904 to January 2nd, 1905) showcased the value of large-caliber siege artillery and this pushed development of like-minded systems for Germany.
A muzzle-loading design was completed in 25cm (250mm) caliber which featured a rifled barrel for accuracy at range. Recoil was aided through a hydro-spring system. A two-wheeled "box trail" carriage was used to bring the weapon into play; the actual launcher dismounted when made ready to fire. Its metal construction meant that the system weighed some 1,700lbs and required multiple crew in her operation and movement (as well as beasts of burden for her road transport). The launcher unit was given an elevation span of +45 to +75 degrees with traversal limited to 12 degrees to each side. The standard HE (High Explosive) shell was 110lbs with a heavier shell weighing 210lbs also being offered for more "stubborn" targets. Charges were loaded separately for each projectile by way of four disks. Muzzle velocity was low at 660 feet per second while effective range was out to 585 yards with a maximum reach of 1,000 yards. Sighting was through a panoramic vision assembly. Two barrel lengths were eventually developed - the original "a/A" L/3 model at 2 feet, 6 inches long and the n/A 4 L/5 model of 4 feet length.
In practice, the heavy and cumbersome 25cm unit proved highly effective thanks to its heavy shell capability. It held a rather reduced range but allowed for useful in-direct, non-line-of-sight fire which meant that the firing crew was relatively protected when set up in a sunken position such as a trench. Early successes in the field prompted a focus in manufacturing more of the type which led to a spike in general availability. These weapons proved far less expensive and maintenance-heavy than the larger German counterparts such as rail guns and heavy artillery.
In 1916, the New Model 25cm trench mortar was adopted and this brought about use of the longer L/5 barrel (n/A = "Neuer Art" or "New Model") mentioned earlier. The longer, rifled barrel increased overall effective range which, in turn, made for a more potent weapon. This led to the L3-barreled forms being recognized with the "a/A" marking for "Alter Art" ("Old Model").
25cm schwerer Minenwerfers were in use up to the end of the war in November of 1918. Afterwards, much of the German weapon capability was reduced and many forms were scrapped or taken as war prizes/reparations.