'Setting Sun' was a US one of a pair of strategic origins for the 'Matterhorn' bombing campaign against Japan using the growing Boeing B-29 Superfortress heavy bomber strength of General Henry H. Arnold’s 20^th AAF (1944).

'Matterhorn' was developed during October 1943 by Brigadier General Kenneth B. Wolfe, commanding the 58^th Bombardment Wing before his elevation to command of the XX Bomber Command, for implementation by the XX Bomber Command. Wolfe drew on on the earlier 'Setting Sun' plan, which was based on an outline drawn by President Franklin D. Roosevelt at the Casablanca Conference and from the 'Twilight' counter-plan offered by Lieutenant General Joseph W. Stilwell. The most significant difference between the two plans was that 'Setting Sun', developed with the support of Major General Claire L/ Chennault’s China-based 14^th AAF, proposed that the B-29 bombers be permanently based in India and forward-based in China. The forward airbases in China would be self-sustaining on the basis of matériel flown from India over the 'Hump' using B-29 aircraft, a fleet of Consolidated C-109 fuel tankers, 20 Consolidated C-87 transports, and three Curtiss C-46 Commando bomber support squadrons. Under 'Setting Sun', the forward operating bases for the B-29 bombers were to be in the Guangxi province of southern China, but because of intense Japanese pressure against the ground and air forces commanded by Stilwell and Chennault, the 'Matterhorn' plan moved the bases farther inland to Chengdu province.

Arnold approved the plan on October 12 and presented it to the US Joint Chiefs-of-Staff Committee after convincing Roosevelt, via General George C. Marshall, the US Army chief-of-staff, that no other strategic bombing of Japan was possible until the capture of the Mariana islands group, which had not yet even been scheduled, had been accomplished. Roosevelt was unhappy with the projected start date of 1 June 1944, having promised Generalissimo Chiang Kai-shek, the Chinese nationalist leader, that the campaign would begin 1 January 1944, but agreed on condition that the campaign be continued for a year.

The 20^th AAF was divided into two commands, namely Wolfe’s (from 6 July 1944 Brigadier General LaVern G. Saunders’s and from 29 August 1944 Major General Curtis E. LeMay’s) XX Bomber Command operating from Indian and Chinese bases, and Brigadier General Roger M. Ramey’s (from 28 August 1944 Brigadier General Haywood S. Hansell’s) XXI Bomber Command operating from bases in the Mariana islands group. The offensive was planned in concert with the activities of the US Navy’s large fleet of submarines in the Pacific, which was strangling the maritime lines of communication over which Japan had to import the raw materials and fuel needed for her war effort; the major objective of the air offensive was thus to strike with great precision at war-making industries and communications in Japan, so that even those limited imports that got through the submarine net could not be turned into aircraft, ships, tanks etc.