Caldwell's first, brief combat posting was a British Hurricane unit, No. 73 Squadron, Royal Air Force, in the early stages of the North African campaign. He had gained only a few operational hours when he was transferred to No. 250 Squadron RAF as it converted to the new P-40 Tomahawk, one of the first units in the world to operate P-40s. According to some accounts, on 6 June 1941, Caldwell as Flying Officer Jack Hamlyn's wingman, was involved in the P-40's first ever kill, of an Italian CANT Z.1007 bomber, over Egypt. However, the claim was not officially recognised. (Hamlyn and Sergeant Tom Paxton scored the first official kill two days later, another CANT.) Soon afterwards, Caldwell served with the squadron over Syria and Lebanon.