In 1938 the Japanese bombers maintained their absolute supremacy over the CAF, which operated only 3 depleted twin-engined bombardment squadrons among its inventory. For obvious reasons, the CAF had to carry out its defensive strategy wisely. Its most historical and daring action taken was the Operation Humane Expedition to call up the self consciousness of Japanese civilians. Three years before Doolitle's air raid over Tokyo, 2 Martin 139WC bombers of the 14th BS/2nd BG performed a successful leaflet drop on southern Japan.
On the night of May 19/20 under the Sqn CO Maj. Xu Huan-Sheng (徐煥昇), s/n 1403 and 1404 were spread out in a network of home base Hankou, forward base Ningpuo, and 7 navigation outposts. They reached the enemy coast line in the dusk and released leaflets over Nagasaki, Fukuoka, Kurume, and Saga, also reconnoitered air bases and harbors below. For the poorly-equipped CAF at the time, it was a complicated risk. The importance of this "psy-ops" was the boost which the humanitarian mission gave to the Chinese; surely the “consciousness” of the trigger-happy warmonger was untouched.
Endured years of hardship and penny-pinching expenditures, the CAF BSs fought in a limited way by adapting to e.g. Martin 139WC (export version of B-10B, "C" was for China), Heinkel He-111, Henschel Hs-123, Curtiss Shrike (export version of A-12), and Vultee V-11G bombers to meet the needs of the next emergency.