Up to around 1400 the largest guns on record whether forged from iron or cast from 'brass' did not exceed five inches (12.7cm) in calibre.These pieces were useful in the anti-personnel role only; against fortified works they were comparatively ineffective.

Now mediaeval warfare frequently involved sieges of castles or other fortified works. When confronted with such a work the attacker could choose: either surround it and starve the occupants into submission, or make a breech in a wall, send in a storming party, and settle the issue by combat. If time was short - and the garrison had provisions enough to prolong the outcome - the second choice was often preferable. The old mechanical 'engines of war', eg ballista and catapulta, the ancient equivalents of the gun and howitzer respectively, were ponderous and complicated contraptions, slow to set up and slower still to make an impression on heavy masonry. On the other hand the gun was comparatively simple to bring into action and man - if only its power could be increased. So early in the 15^th century leading military men who were artillery-minded directed their gunmakers to build battering pieces capable of punching holes in stone walls. The same men who forged the smaller wrought iron bombards found making the heavier no more difficult. Indeed in some respects they found them easier for their size provided more 'hammer room'.

To achieve the desired destructive effect with the low muzzle velocities mediaeval powder produced, the stone roundshot the Gunners were obliged to use had to be heavy and consequently of large calibre. Some of the great bombards constructed were large even by modern standards. Calibres up to 36 inches (91cm) are recorded.