Chennault's Flying Tigers began training in August 1941 and fought the Japanese for six months after the attack on Pearl harbor. Chennault took two squadrons to Kunming, China, which became his primary base of operations.

Chennault's three squadrons used P-40s to guard the Burma Road, Rangoon, and other strategic locations in Southeast Asia and Western China against Japanese forces, as China had few, if any, modern planes. President Roosevelt diverted 100 new P-40's slated for delivery to Britain to China for Chennault's use. He also authorized Chennault to recruit pilots from the USAAF ranks.

The Flying Tigers existed as a combat unit for only seven months. They never had more than 50 combat ready aircraft at a time, and never more than 70 pilots ready to fly. They faced an enemy force that was 20 times larger with better airplanes. They were chronically short of parts and supplies. Nevertheless, they shot down at least 10 Japanese airplanes for every one they lost, and they held the line in China until the regular Army Air Forces could get there.