The US ran several air raids on Japan that relied on China as support, either as a landing area or a crashing area with crews being rescued by Chinese people. Were the nationalist Chinese and the Communist Chinese agreed to just let Americans be dropping bombs on the Japanese?
I answered a question about US air bases in China just a few days ago that might interest you. The Americans did use airfields in China that were controlled by Chiang Kai-shek's nationalist forces for bombing missions against Japan, and there was also some cooperation with the communist forces to assist downed airmen. However, American bombers did not operate out of communist-controlled airfields. Given the logistical difficulties of supplying the bombers in the theater and the airfields at Chengdu, there was little reason to send more bombers to operate in the region.