On the outbreak of World War II, Purvis was appointed by the British government to be director-general of the British Purchasing Commission, which was charged with buying war supplies from the United States. He was also chairman of the Anglo-French Purchasing Board, where he worked alongside Jean Monnet. In June 1940, when France was on the verge of concluding an armistice with Germany, Purvis arranged to take over all the pending French weapons contracts in the United States at the cost of $612 million. Purvis negotiated for the British during September 1940 to secure items like torpedo-boats, aircraft, and munitions that the United States would provide the British in addition to the 50 destroyers as part of the Destroyers for Bases Agreement.