CAMD RF 81-12104-38
"To the Deputy People's Commissar of Defense and Head of GAU KA Marshall of the Soviet Union, comrade Kulik.
In 1940, factory #221, based on AU's tactical-technical requirements, developed a project of a 450 mm howitzer, which was approved by ZhAK #0012-40g.
In the same year, the factory developed a technical project, which was examined and approved for the production of working blueprints.
The journal for examination of this technical project is composed, but still not approved by you.
The factory, not waiting for approval, began, and has almost finished, the creation of working blueprints.
The UVNA GAU plans for 1941 include an experimental 450 mm howitzer, but due to the unconfirmed nature of the situation, the contract has not been signed and production has not started.
I am reporting this, and waiting for further instructions on whether or not to sign a contract in the current year for the production of a 450 mm howitzer prototype and ammunition for it.
Chief of the UVNA GAU KA
Military engineer 1st grade, Lipin."
The document is dated February 1st, 1941. There was not enough time before the start of the war to develop such a massive device, but a model was built.
And here's some more technical data:
The artillery unit is transported on 7 25 ton railroad carts (the total mass of the system was "only" 110 tons). The rate of fire was 1 shot every 5 minutes. The gun fired two shells: one concrete penetrating (projected to penetrate 3 meters of reinforced concrete at 9.5 km), which weighed 1500 kg and contained 150 kg of explosives, and an HE grenade (projected to create a crater 230 cubic meters in size), which weighed 1060 kg and contained 159 kg of explosives.