The Cappellini was launched in 1939 and differed from the first nine of the class only in her diesel engines, which were made by Fiat instead of CRDA.
In August 1940 she was sent, along with others of the class, to the Atlantic to operate from Bordeaux.
The Cappellini sank the armed steamship Kabalo with its deck gun on 15 October 1940. The 26 surviving crew were taken on board and disembarked at Santa Maria in the Azores.
On 5 January 1941 the Cappellini sank the British steamship Shakespeare after an artillery duel in which Sergeant Ferruccio Azzolin was killed. Shakespeare's 22 surviving crew were taken on board and disembarked at Cabo Verde.
The Cappellini sank the armed steamship Eumaeus with torpedoes and its deck gun on 14 January 1941. The artillery duel lasted for two hours in which Sergeant Francisco Moccia, Common Giuseppe Bastino, and T.G.N. Danilo Stiepovich were killed. The submarine was damaged both in the duel and from bombs dropped by Allied aircraft, and the wounded were disembarked at the Spanish port of La Luz, in Gran Canaria.
The Cappellini sank the Swedish motor ship Tisnaren on 19 May 1942, all 40 crew being later rescued.
On 31 May 1942 the Cappellini torpedoed and sank the British fleet tanker Dinsdale.
In the central Atlantic Cappellini took part in rescuing survivors from the British liner Laconia which had been sunk by the German U-156 on 12 September 1942. This episode, known as the Laconia incident, changed the German attitude towards rescuing survivors of torpedoed ships.
In 1943, after actions in the Atlantic had caused further damage, the Cappellini, along with six other Italian submarines, was converted into a transport submarine at the request of the Germans, in exchange for seven new Type VIIC U Boats. Her name was changed to Aquilla III at the same time. She sailed for Japan on 11 May 1943, reaching Sabang with its fuel tanks empty on 9 July, and Singapore the next day.