The ships were ordered in 1926 and were based on the British County-class cruisers. The designer was Sir Phillip Watts. These ships had anti-torpedo bulges and a slightly narrower beam as well as more powerful machinery. The boiler rooms were re-arranged compared to the British design and trunked into a single massive funnel rather than three funnels used on the British cruisers. The Spanish ships had a stronger secondary armament and although the original design included a catapult for a seaplane this was never installed.
Both ships were built by SECN in Ferrol
A third ship was projected but not ordered.
Both ships were making their final sea trials when seized by the Nationalist side in the Spanish Civil War. Both of them were fitting out and completed with makeshift fire control and secondary armament.