By mid-1940, the IJN G3M Model 21 bombers that formed the primary aircraft of the carpet-bombing campaign against Chonqing and Chengdu, have become largely relegated to nighttime bombing attacks while daylight raids were being replaced with the new and improved Model 22/23 G3Ms equipped with the higher-octane rated and supercharged Kinsei 51 engines, boosting the power of each of the twin-engined bombers by 500 horsepower and greatly enabling the "schnellbomber" strategy further above the general obsolescence of the defending Chinese fighter aircraft now struggling to gain speed and altitude to make even a single head-on pass against the high-speed/high-altitude IJN bombing runs, and even more vulnerable to the massed-firepower of those heavy bomber formations.