In times of war, Packard turned its automotive expertise to the production of thousands of trucks and Liberty aircraft engines in World War I. In World War II, Packard's "Master Motor Builders" produced engines that powered Lancaster bombers, Supermarine Spitfires, Mosquito fighters, P-51 Mustangs, and the island-hopping PT boats in the South Pacific. During World War II, Chrysler Defense Engineering leased the Proving Grounds to test tanks and other tracked vehicles before automotive testing resumed there after the war, and during that tank-testing work it built the large testing building behind the Proving Grounds' Repair Garage.