'Twilight' was the US version of 'Setting Sun' proposed by Lieutenant General Joseph W. Stilwell, chief-of-staff to Generalissimo Chiang Kai-shek, the Chinese Nationalist leader, and adopted by the Combined Chiefs-of-Staff planners as more practical than 'Setting Sun' (1944).

Suggested by the USAAF, 'Setting Sun' had proposed the establishment on Chinese bases near Changsha of a force of between 10 and 20 (rising eventually to 30) groups of Boeing B-29 Superfortress heavy bombers, supported initially by 2,000 and later 4,000 Consolidated B-24 Liberator heavy bombers converted into transports to ferry supplies from India. The air force planners estimated that if five missions per month were carried out by 28 B-29 groups, Japan could be brought to its knees in six months or, more realistically, that a campaign started in October 1944 with smaller numbers (but increasing as B-29 production permitted) could permit an Allied occupation of Japan from September 1945.

Evolved by Stilwell with Major General George E. Stratemeyer (commanding the Eastern Air Command, South-East Asia Air Command), 'Twilight' suggested that the transport aircraft component be scaled down and that the B-29 bombers be based near Calcutta to shuttle forward for operations to bases near Kweilin. At these advanced bases the bombers would offload excess fuel and take on bombs. The 'Twilight' plan ensured greater security for the bombers, as well as a larger measure of self-sufficiency, and would have made it possible for 10 groups to began operations by April 1945. The disadvantage of 'Twilight' compared with 'Setting Sun' was that Kweilin is 200 miles (320 km) farther than Changsha from Japan.

In the event 'Twilight' was superseded by 'Matterhorn' based on Chengtu, some 400 miles (640 km) farther from Japan, the US reasoning being that the necessary bases could be readied in 1944 rather than 1945.