Among the changes were the introduction of a partially double hulled design, rearrangement of the ballast tanks, increases in fuel capacity and range, and a strengthening of the armament.

In 1937, along with many other submarines of the Royal Italian Navy (Regia Marina), Galileo Galilei secretly participated in the Spanish Civil War, conducting several missions without achieving any success.

Following the Iride-HMS Havock incident, in September 1937 the Nyon Conference was called by France and Great Britain to address the "underwater piracy" conducted against merchant traffic in the Mediterranean.

On 14 September, an agreement was signed establishing British and French patrol zones around Spain (with a total of 60 destroyers and airforce employed) to counteract aggressive behavior by submarines.

Under pressure from Franco's regime, Italy decided to transfer 4 more submarines (in addition to Archimede and Evangelista Torricelli already being operated by the Falangists) to the Spanish Legion (Legión Española or Tercio de Extranjeros).

She was placed under the direct command of Spanish admiral Francisco Moreno, was renamed General Mola II and assigned pennant number L1.

However, Galileo Galilei retained her commander (captain Mario Ricci), senior officers and Italian crew, but they had to wear Spanish uniforms and insignia.

The other three Italian submarines transferred to Tercio were Onice (Aguilar Tablada), Iride (Gonzalez Lopez) and Galileo Ferraris (General Sanjurjo II).

[4] It is likely, the explosions were heard in Aden and the smoke column rising from the burning tanker was also observed, but no British ships or planes appeared and the submarine continued her mission unmolested until the afternoon of 18 June when a Yugoslavian steamer Drava was spotted.

When the darkness fell, the boat resurfaced to recharge the batteries, but it was discovered by the British ship forcing the submarine to crash dive and go through a brief but intense depth-charge attack which did not cause any damage.

In the morning of 19 June, while Galileo Galilei was laying immobile on seabed, the first mild symptoms of methylchloride poisoning appeared in some crew members.

As the fight began, the bow gun's sighting mechanism on the Galileo Galilei failed, greatly affecting the accuracy of shooting.