They were in all essential points improved versions of the Ajax class, with slightly greater displacement, protection and speed, with better handling characteristics, and most significantly, with breech-loading artillery replacing the obsolescent muzzle loaders.

The class was originally designed to carry the then-standard muzzle-loading artillery, while European navies had already for several years been equipped with breech-loading guns.

In 1879, while the Colossus class was under construction, a 100-ton muzzle-loader was being tested at the Royal Arsenal, Woolwich, and the 16-inch 80-ton guns destined for HMS Inflexible were nearly complete.

Responding to a groundswell of professional opinion, the Admiralty sent a group of senior officers to witness and report on the performance of the new breech-loading weapons being produced by Krupp in Essen.

The Colossus class was nominated to be armed with these guns; the delay in their manufacture largely accounts for the protracted building times of these vessels.