Slessor was closely involved in planning the combined Allied air offensive in Europe. At the Casablanca Conference in January 1943, he was able to influence Britain's Secretary for Air, Sir Archibald Sinclair, and Chief of the Air Staff, Air Chief Marshal Sir Charles Portal, to agree to USAAF proposals that led to a "round-the clock" bombing policy against Germany, with the US mounting daylight precision attacks and the RAF conducting area bombing at night. Slessor's assigned personal pilot was Flight Lieutenant Owen Phillipps DFC, an Australian from No. 14 Squadron RAF and a distinguished veteran of the Mediterranean conflict.