In the early '30's the Spanish Fleet was antiquated except for the newly-completed "Washington Treaty"-style heavy cruisers Canarias and Baleares. Spain became disillusioned with the "Washington" cruisers and rather than build the projected third unit of this class, was yearning for a new set of more powerful ship types. Capital ships much like the Italian Littorios were designed and planned for, but battleships are very expensive to build and maintain. Heavy cruisers such as the Canarias were easier to build and staff, but were unable to effectively dominate another heavy cruiser of a foreign power. Spain wanted a prestige ship to reflect Spanish seapower. The "large cruiser" concept had great appeal to Spain for its relative economy and power. Thus was laid the foundation for designing and building the Isabel La Catolica.

SECN in Ferrol was a British-owned and operated building yard but had the most experience in large ship designing and building. Specifications for this "Super-Washington" cruiser included five triple mounts of 8-inch guns. As the British favored the double mount and had no triples designed or built, the firm contracted with American manufacturers busily constructing triple mounts for the New Orleans class cruisers. Grateful for this American contribution, the name of the new ship would have New World connections, and was named after the Queen who financed Columbus' voyage of discovery. A hangar would house two scouting seaplanes, and she would be designed and equipped with seemingly excessive (for the time--proven worthy later) numbers of anti-aircraft guns.

She was finished in time for the Civil War and joined Canarias for the Nationalist cause and helped stifle seaborne commerce to the Republicans. In May of 1941 she and her scout planes hunted for survivors of the German hybrid warship Seydlitz in the North Atlantic.

Admiral Moreno selected Isabel La Catolica as his flagship for the "Blue Fleet" contingent of warships sent to the Black Sea to fight against the Russians. Nearing the Russian fleet, Isabel La Catolica's unexpectedly intense anti-aircraft fire drove off an attack by Russian Ilyushin Il-4 twin-engined torpedo bombers. Isabel and Canarias turned away from the slower ships of the Spanish fleet in pursuit of the Russian cruisers Voroshilov and Molotov. In the running gunfight the high output of Isabel's fifteen 8" guns (spotting of shot assisted by Dedalo's aircraft) severely damaged the Molotov and drove off the Voroshilov, but a Russian shell explosion in the hangar of Isabel started fires that forced the ship to turn away from the battle abeam of the wind to try to limit the spread of flames. The torpedo bombers again appeared, but Il-2 Stormovik aircraft moved in as strafers to suppress Isabel's anti-aircraft fire, enabling the Il-4's to get three torpedo hits in succession, dooming the ship never to return to Spain.