From Lexicanum

The Imperial Bombard is the heaviest artillery piece in common use by the Imperial Guard, and is designed for cracking open fortresses. Due to its enormous weight, it is based on a stripped-down Leman Russ chassis rather than the lighter Chimera hull used for most self-propelled artillery. Even with the Russ' heavier engine and tread suspension, it is very slow. It is armed with a Heavy Siege Mortar.

Chassis Dispute

There is some confusion over what chassis the Bombard is based on. The original Epic miniature for this vehicle seems to be based on the Chimera hull. However, the Forgeworld model for Warhammer 40,000 is clearly based on the Leman Russ, and is described as such in Imperial Armour 3. It's possible that the Bombard mortar can be mounted on either hull.

Sources

• Imperial Armour Volume III
• Epic Living Rulebook

Imperial Guard Vehicles
Leman Russ Chassis	Leman Russ Battle Tank • Leman Russ Demolisher • Leman Russ Conqueror • Leman Russ Exterminator • Leman Russ Executioner • Leman Russ Vanquisher • Destroyer Tank Hunter • Thunderer • Bombard • Atlas
Chimera Chassis	Chimera • Hellhound • Basilisk • Griffon • Medusa Siege Tank • Salamander Command • Salamander Scout • Hydra Flak Tank • Manticore • Trojan
Other	Sentinel • Cyclops • Siegfried • Centaur
Super Heavy Tanks	Baneblade • Stormblade • Shadowsword • Stormsword • Stormhammer • Gorgon Armoured Assault Transport