Slayton moved to Columbia Army Air Base in South Carolina for the three-month-long B-25 Mitchell training. After completing training, he was assigned to the 340th Bombardment Group, and departed for the European theater of operations on ship from Newport News, Virginia. After a stop in Zerni, North Africa, his convoy travelled to Naples, Italy. While traveling near the Strait of Gibraltar, their ships came under attack from German bombers and submarines.: After he arrived in Naples, the 340th Bombardment Group moved to San Petrazio, where Slayton flew combat missions into the Balkan Peninsula. After six weeks, he moved to Foggia, where 48 aircraft were destroyed after an eruption of Mount Vesuvius. Afterwards, Slayton flew out of Salerno and Corsica, where he upgraded from copilot to pilot. After 56 combat sorties, Slayton completed his combat tour and returned to the U.S. in May 1944.: