Knowing that the 'M' class ships would have a limited shelf life, the contract for the Serrano class was initiated 5 years after the last of the 'M's had been received by Chile. 6 were ordered to the winning bidder Thornycroft Yard at Woolston. Purchased at the rate of two per year, the orders for more destroyers of the class were cancelled in favour of the battlecruisers Capitan Prat and Coquimbo. A new type was laid down in 1938.
The design was similar to the new A-B class destroyers under construction for the Royal Navy. The ships were to be used in mainly cold weather conditions and there accommodation for crew took this into consideration. On completion, the first two ships joined the Atacama in Force Magellan, guarding the straits. On the completion of the first two Almirante Riveros class destroyers, the first two of this class were taken in hand for a major upgrade refit in Australis. The second pair had just completed their upgrades and were ordered to join the Allied forces that were to be used to retake the Falkands Islands. The last pair would go north to a US refit yard for a basic refit but the low angle 4.7" would remain their main armament.
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The change to twin 4" made an appreciable difference to the ships capabilities. Tied to the dual purpose director with its early radar unit, the 4" were far more accurate. The anti-submarine capabilities also increased during this period with bigger depth charge racks and depth charge throwers making their appearance. At Chilean insistence the ships kept their two triple sets of torpedo tubes.
