Considering that I've been housebound this week, I've had a chance to catch up on the History channel. A few days ago they had a 4-5 hour series of docos (Australian made) on the battleship through the ages.
They consider the first "true" ship of the line - purpose built man'o war to be the Mary Rose.
The series goes on to describe the main line ships through the steam age.
The proposition is made that the Dreadnought came about as a review of the Japanese victory over the Russians in the Battle of Tshushima (sp?) Straits - 1905 - where the heavy guns (12-in) did practically all the damage, the smaller guns were never in range. Therefore the decision was made to equip Dreadnought with all large guns (10 x 12-in ?) which "instantly" made all other battleships outmoded.
The doco also showcased the Battles of Dogger Bank and Jutland to some detail, that last major ship to ship battle, since WW2 basically relegated the battleship to a support gunship, rather than the main striking/battle component.