this is a model of the Little David Heavy mortar designed to be used during the invasion of Japan during WWII. It has a calibre of 36in and designed to break through bunkers and stationary defenses having to have its base buried so it could handle the recoil. there are two versions one which is based on the actual mortar and the other designed as a self-propelled unit

the vehicle used is a t-35 by HAPPYBATTLESHEEP
https://www.thingiverse.com/HappyBattleSheep/designs