When the German conquest of Europe in World War 2 was being drawn up, its warplanners recognized the need for dedicated siege artillery to break the strong line of defensive-minded fortresses between Berlin and its enemies in the West. The charge for such weaponry fell primarily to two of the renowned heavy steel manufactures in Krupp and Rheinmetall. The latter designed and developed the 35.5cm Haubitze M1, a massive super-heavy gun that appeared before the fighting of World War 2 and would see action in several of its notable siege actions including Leningrad. This new weapon fired a large projectile out to 22,800 yards and held the power to destroy most any fortified structure at range. Its value was such that it is believed a total of seven or eight guns were constructed with first delivery taken in 1939.
With a German Army request received in 1935, design work began in 1936 during the height of German rearmament. Authorities requested a dimensionally larger version of the 24cm Kanone 3 (detailed elsewhere on this site) to which Rheinmetall engineers drew up plans for a new super gun utilizing much of the same general form and function of the original which (was still in development). Hydropneumatic systems controlled the violent recoil effect through a "dual recoil" arrangement in which the gun tube and gun mounting recoiled individually atop a static firing platform. The large gun system could be broken down into six smaller (though still heavy) loads for "easier" transport by accompanying mover vehicles (typically half-tracks). The carriage system itself was of a two-piece design for portability. Among the support components attached to the M1 system was a generator-powered crane that allowed for disassembly and reassembly of the weapon. The generator also served in the elevation of the gun tube when in action (manual override/backup also possible). The elevation span was +45 to +75 degrees.
The 355mm shell could pierce concrete-type fortifications through a penetrator round weighing 2,042 pounds and using as many as four propellant charges. A High-Explosive (HE) projectile was also available for anti-personnel service and these weighed 1,270 pounds. With these weights in play, the operating crew consisted of multiple men and reloading required the services of an ammunition hoist. During peak usage, an M1 crew could expect a rate-of-fire no greater than one round in four to five minutes. Muzzle velocity reached 1,900 feet per second.
M1 guns were used in the assault on Belgian and French fortresses during the German march to the English Channel and Paris respectively. The guns were then transport to serve along the Eastern Front during and after Operation Barbarossa - the German invasion of the Soviet union that began in June 1941. The guns provided heavy long range bombardment on Soviet positions at the successful Siege of Sevastopol before concentrating fire on enemy elements during the Siege of Leningrad. These guns were then brought to bear on the hapless Poles during the ill-fated "Warsaw Uprising" of 1944.
Believed to have survived in service into 1945, it is believable that the weapon grew to become more and more tactically limited as the war progressed in favor of the Allies. Such siege instruments were a liability due to their size, weight, and manpower commitment considering the battlefield results recorded. No doubt their shells could pulverize static, fortified structures with relative ease but their range left something to be desired and the return-on-investment proved questionable as the German war effort went on the defensive.