'Matterhorn' was the US fully developed version of 'Drake' (ii) for the bombing of targets including the Japanese home islands and Japanese bases in China and South-East Asia, implemented with the arrival of Major General Kenneth B. Wolfe’s XX Bomber Command of the 20^th AAF in China after its activation on 4 April 1944 in response to the directive of the 'Sextant' conference at Cairo for the strategic bombing of Japan (15 June 1944/March 1945).

The concept of basing Boeing B-29 Superfortress heavy bombers in China first emerged at the 'Symbol' conference in Casablanca during January 1943, and even as planners assessed this option, the Combined Chiefs-of-Staff, meeting at the 'Quadrant' conference in Quebec during August of the same year, authorised a strategic offensive to the west through the central Pacific to include the seizure of the Mariana islands group: these were closer to Tokyo than any bases in China, and also had the advantage that, in Allied hands, could be supplied and defended more easily than bases in other locations. In September, the Combined Chiefs-of-Staff planning team concluded that B-29 bombers operating in China, and therefore isolated from resupply except by inclusion in the 'Hump' airlift, would suffer from a mass of logistical problems. President Franklin D. Roosevelt decided in favour of bases in China, however, as he was impatient to start the bombing of the Japanese home islands and wished to bolster the Chinese war effort. At the 'Sextant' conference Roosevelt promised Generalissimo Chiang Kai-shek, the Chinese nationalist leader, that B-29 heavy bombers would be based in China. General Henry H. Arnold, the USAAF’s commanding general, supported that decision as a temporary expedient, but knew that strategic missions against Japan from the Mariana islands group offered greater strategic capabilities once bases in these islands had become available.

'Matterhorn' was developed by Brigadier General Kenneth B. Wolfe of he USAAF in October 1943 for implementation by the XX Bomber Command. Wolfe drew on an initial plan codenamed 'Setting Sun' and based on an outline drawn by Roosevelt at the Casablanca conference and from the 'Twilight' counter-plan offered by Lieutenant General Joseph W. Stilwell, the chief-of-staff to Chiang. The primary difference between the two plans was that 'Twilight', as originally proposed by Major General Claire L. Chennault, commanding the US 14^th AAF in China, proposed that the B-29 bombers be based in India and stage through Chinese airfields. The forward air bases in China would be self-sustaining, supplied from India by flights over the 'Hump' using B-29 aircraft, a fleet of Consolidated C-109 fuel tanker aircraft, 20 Consolidated C-87 transport aircraft, and three Curtiss C-46 Commando bomber support squadrons. In 'Setting Sun', the forward bases were to be in Guangxi in southern China, but because of intense Japanese pressure against the forces commanded by Stilwell and Chennault, the 'Matterhorn' plan moved the bases farther inland to Chengdu.

Arnold approved the plan on October 12 and presented it to the US Joint Chiefs-of-Staff 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, an undertaking which had not yet been scheduled. Roosevelt was unhappy with the projected start date of 1 June 1944, having promised Chiang that the campaign would begin on 1 January 1944, but he agreed on condition that the campaign be continued for a year.

The key development for the bombing of Japan was the B-29, which had an operational range of some 1,800 miles (1895 km), and eventually delivered almost 90% of the bombs dropped on the Japanese home islands.

As the planning of the B-29 campaign was developed, the only three basing options currently available were at Darwin in northern Australia, Ceylon, and Kharagpur in north-eastern India. From Darwin the bombers could have attacked targets in a semi-circle extending as far to the east as the Solomon islands group, to the north as far as the southern half of the Philippine islands group, and as far to the west as the eastern end of the islands of Borneo and Sumatra in the Dutch East Indies; however, there were few strategic targets, other than oil production, in this arc. From Ceylon the bombers could had attacked targets in a narrower arc from Sumatra in the Dutch East Indies northward to much of South-East Asia; again there were comparatively few strategic targets, except oil production, in this arc. From Kharagpur the bombers could have attacked targets in an arc between the north-western end of Sumatra to south-eastern China; again there were comparatively few strategic targets in this arc.

Possible future basing options were China, the Mariana islands group and the Philippine islands group, though the latter had still to be taken. From bases in the area of Chengtu in China, the bombers to attack targets over a large arc between southern Manchukuo and Korea in the north, via Formosa and the northern part of the Philippine islands group to the eastern part of South-East Asia in the south. From the Mariana islands group the bombers could also attack targets in a wide arc from the southern part of the Japanese home islands in the north, via Formosa to the Philippine islands group, to the eastern part of the Dutch East Indies. Finally, from the Philippine islands group the bombers could attacks targets in a large arc extending from the southern part of the Japanese home islands in the north, much of Korea, most of eastern China, most of South-East Asia and the northern part of the Dutch East Indies.

The basing options which offered the greatest strategic opportunity was the Mariana islands group, but this was still in Japanese hands and the choice for the start of the B-29 campaign thus went to China.

The USAAF’s advance echelons for 'Matterhorn' arrived in India in December 1943 to organise the building of airfields in India and China. Many thousands of Indians laboured to construct four permanent bases in eastern India around Kharagpur, and at much the same time and about 1,000 miles (1610 km) to the north-east, across the Himalayan mountains, about 350,000 Chinese laboured to build four staging bases in western China near Chengdu. The original plan envisaged that this would be sufficient for two combat wings each with of 150 bombers, but in April 1944 the second of those wings had not been sufficiently organised, equipped or trained for operational deployment, and Brigadier General Kenneth B. Wolfe’s (from 6 July 1944 Brigadier General LaVern G. Saunders’s) XX Bomber Command was reduced to a single bombardment wing, fatally crippling its ability to sustain itself by airlift. By April 1944, the four B-29 groups of the 58^th Bombardment Wing were available in Asia, and eight operating bases had become operational.

Th 58^th BW comprised Colonel Leonard F. Harman’s 40^th Bombardment Group based at Chakulia airfield in India with Hsinching airfield (A-1) in China as it forward base; Colonel Henry R. Sullivan’s 444^th Bombardment Group at Dudhkundi airfield in India and Kwanghan airfield (A-3) in China; Colonel Richard H. Carmichael’s 462^nd Bombardment Group at Piardoba airfield in India and Kuinglai (Linqiong) airfield (A-5) in China; and Colonel Howard E. Engler’s 468^th Bombardment Group at Kalaikunda airfield in India and Pengshan airfield (A-7) in China.

To avoid the risk that the capabilities of the B-29 might be wasted on the tactical support of land operations when they had been designed for the much more useful strategic role against targets in the Japanese home islands, in April 1944 the Joint Chiefs-of-Staff approved the establishment of the 29^th AAF and thereby completed the development of 'Matterhorn'.

Acting as executive agent for the Joint Chiefs-of-Staff, Arnold named himself as the 20^th AAF’s commanding officer and the AAF Air Staff as the Air Staff of the 20^th AAF. Brigadier General Haywood S. Hansell served as the 20^th AAF’s chief-of-staff and was the de facto commander of the 20^th AAF after Arnold had suffered a heart attack in May 1944. Centralised control of the Superfortress force from Washington also served to mark the recognition of the B-29 as a strategic weapon with a capability which transcended theatres and services.

In this same month, the first Superfortress bombers reached India, after flying across the Atlantic Ocean using the South Atlantic Transport route from Morrison Field, Florida to North Africa via Natal, Brazil, and thence to Arabia, Persia and finally India. Accompanying the bomber was Wolfe, the designated commander of the XX Bomber Command, which had been reassigned from Major General Uzal B. Ent’s 2^nd AAF as the operational component of the 20^th AAF. Although a highly capable officer in research and development, with excellent technical knowledge of the B-29, Wolfe had no senior command or operational experience (between June 1937 and June 1943 he had been the US Army Air Corps' representative at the Douglas Aircraft Company of El Segundo, California, the Assistant Chief of the Production Engineering Section, Air Corps Material Division, the Chief of the Production Engineering Section, Air Corps Material Division, and the Chief of the Production Division, Material Center), and was thus an unfortunate choice to head the first B-29 operations. The headquarters of the 58^th Bombardment Wing also arrived in India during the spring of 1944.

A committee of operations analysts who advised the Joint Chiefs-of-Staff and the 20^th AAF on targeting matters recommended Superfortress attacks on the coke ovens and steel factories in Manchukuo and Kyushu, the southernmost main island of the Japanese home islands as the destruction of these key industries would severely cripple the Japanese war effort. Also high on the B-29s' target list were important harbour facilities and aircraft factories.

Wolfe launched the first B-29 combat mission on 5 June against Japanese railway facilities at Bangkok in Thailand, about 1,000 miles (1610 km) distant. Of the 98 bombers which took off from India, 77 attacked their targets with 368 tons of bombs. Encouraged by the results, the XX Bomber Command prepared for its first raids against Japan.

On 15 June, 68 B-29 bombers took off at night from staging bases at Chengdu to bomb the Imperial Iron and Steel Works at Yawata on Kyushu, more than 1,500 miles (2415 km) distant. This was the first US air raid on the Japanese home islands since 'Conceal' (the 'Doolittle raid') of April 1942, and marked the start of the US strategic bombardment campaign against Japan. Like the Doolittle attack, the raid of 15 June achieved little physical destruction: only 47 of the 68 aircraft bombed the target area; four aborted with mechanical problems, four crashed, six jettisoned their bombs because of mechanical difficulties, and the other three bombed secondary targets or targets of opportunity. Only one B-29 was lost to Japanese aircraft.

The second full-scale attack followed only on 7 July, and by this date Arnold, impatient with Wolfe’s progress, had replaced him temporarily with Saunders, until Major General Curtis E. LeMay arrived from Europe on 29 August to assume permanent command. Unfortunately, the three-week delay between the first and second missions reflected the many serious problems that were to prevent the launch of a sustained strategic bombing campaign from China against Japan. Each mission consumed tremendous quantities of fuel and bombs, which had to be shuttled from India to the bases in China over the huge impediment of the Himalaya mountains Each B-29 combat mission required an average of six return cargo missions over the 'Hump', using both tactical aircraft and B-29 bombers modified as fuel tankers. When it was immediately apparent that the operation could not be self-sustaining, the India-China Division of the US Air Transport Command was summoned to support 'Matterhorn' with allocations on its 'Hump' airlift using aircraft diverted from the allocation intended to support the 14^th AAF already in China. In September 1944 70 C-109 tanker aircraft, flown by surplus B-29 crews, were added to the effort, but the XX Bomber Command was concerned lest such aircraft be diverted to other USAAF elements and successfully resisted all attempts to have these aircraft operated by the India-China Division. Its transport procedures contradicted those of India-China Division, however, and as this limited its efficiency, from November 1944 its B-29 aircraft were withdrawn from the airlift and the C-109 aircraft transferred to the India-China Division. With plans already developed to shut down the B-29 force’s forward basing in China at the end of January 1945, the India-China Division took over the logistical supply of bases in China, too late to provide the volume required to stockpile matériel. In total, 42,000 tons of cargo were delivered over the 'Hump' to the XX Bomber Command between April 1944 and January 1945, nearly two-thirds of it by aircraft of the India-China Division.

The range over which the B-29 missions were necessarily flown presented another problem. Tokyo, in eastern Honshu, lay more than 2,000 miles (3220 km) from the Chinese staging bases, and was out of reach of the B-29. Kyushu, in south-western Japan, was the only island of the major home islands within the 1,600-mile (2575-km) combat radius of the Superfortress.

A very advanced warplane for its time, and despite that a bomber which had been pushed through development at great speed, the B-29 was also still suffering mechanical problems, which grounded some aircraft and forced others to turn back before dropping their bombs. Even the bombers which did reach their target area often had difficulty in hitting their objective, in part as a result of the prevalence of extensive cloud cover and high winds. The despatch of larger formations could have helped compensate for inaccurate bombing, but Saunders did not have enough B-29 aircraft to despatch large formations. Added to this, the 20^th AAF at times diverted its Superfortress bombers from strategic targets for the support of theatre commanders in South-East Asia and the South-West Pacific Area. For these reasons, the XX Bomber Command and the B-29 largely failed to fulfil their strategic promise.

LeMay’s arrival started to breath new energy into the XX Bomber Command. A former 8^th AAF group and wing commander, LeMay had achieved remarkable success with strategic bombing operations in Europe, testing new concepts such as stagger formations, the combat box, and straight-and-level bombing runs. At 38 the youngest major general in the USAAF, he had also revised tactics, tightened and expanded formations, and enhanced training for greater bombing precision, and inaugurated a lead-crew training school so that formations could learn to drop as a unit on cue from the aircraft designated as the lead ship.

During his first two months at the head of the XX Bomber Command, LeMay had little more success than Wolfe or Saunders. The command continued to average only about one sortie per month per aeroplane against the Japanese home islands. When General Douglas MacArthur’s forces invaded the Philippine islands group in October 1944, LeMay diverted his B-29 bombers from attacks of Japanese steel facilities to strikes on Japanese aircraft factories and bases in Formosa, Kyushu and Manchukuo.

Meanwhile, LeMay gained the support of Mao Tse-tung, the Chinese communist leader, who controlled parts of northern China. Willing to help against a common enemy, Mao agreed to assist downed US airmen and to locate in northern China a weather station that would provide better forecasts for the XX Bomber Command’s raids on the Japanese in Manchukuo and Kyushu. Hoping to gain US recognition of his own regime, Mao also suggested that the USAAF establish B-29 bases in northern China like those in Chiang area of control in southern China. LeMay declined, however, because he found it difficult enough to supply the airfields at Chengdu.

LeMay continued to experiment with new technologies and tactics and soon imported to China the incendiary weapons being used by the British against Germany. Late in 1944, however, the Japanese 'Ichi' offensive in China probed toward the B-29 and India-China Division bases around Chengdu and Kunming. To slow the Japanese advance, Chennault’s 14^th AAF asked for raids on Japanese supplies at Hankow, and the Joint Chiefs-of-Staff directed LeMay to hit the city with firebombs. On 18 December, LeMay launched the fire raid, despatching 84 B-29 aircraft to attack from medium altitude with 500 tons of incendiary bombs. The attack left Hankow burning for three days, proving the effectiveness of incendiary weapons against the predominantly wooden urban structures of the Far East.

By a time late in 1944, US bombers were raiding Japan from new bases quickly constructed in the recently captured Mariana islands group, rendering unnecessary continued operations from the vulnerable and logistically impractical China bases. In January 1945, therefore, the XX Bomber Command abandoned its bases in China and concentrated the 58^th Bombardment Wing’s resources in India, signalling the end of 'Matterhorn'. During the same month, LeMay moved to the Mariana islands group to assume command of the XXI Bomber Command on 20 January and leaving command of the XX Bomber Command in India to Brigadier General Roger M. Ramey. Between January and March, the XX Bomber Command’s bombers assisted Admiral the Lord Louis Mountbatten’s South-East Asia Command, supporting British and Indian ground forces in Burma by targeting rail and port facilities in Indo-China, Thailand and Burma. More distant targets included refineries and airfields in Singapore, Malaya and the Dutch East Indies. The 58^th Bombardment Wing, the only operational wing of the XX Bomber Command, remained in India until the end of March 1945, when it moved to the Mariana islands group to become an element of the XXI Bomber Command, and the XX Bomber Command creased to be an operational command.

The US Bomber Summary Survey states that 'Approximately 800 tons of bombs were dropped by China-based B-29s on Japanese home island targets from June 1944 to January 1945. These raids were of insufficient weight and accuracy to produce significant results.' Thus the XX Bomber Command failed to achieve the strategic objectives that the planners had intended for 'Matterhorn', largely as a result of logistical problems, the bomber’s ongoing mechanical difficulties, the vulnerability of Chinese staging bases, and the extreme range required to reach key Japanese cities. Although the B-29 force achieved some success when diverted to support Chiang’s forces in China, MacArthur’s offensives in the Philippine islands group and Mountbatten’s efforts in the Burma campaign, they generally accomplished little more than the smaller Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers assigned to the 5^th, 10^th, 13^th and 14^th AAFs in the Far East.

Chennault always thought that the XX Bomber Command was a liability, and that the supplies of fuel and bombs the command consumed could have been more profitably used by his 14^th AAF. The XX Bomber Command used almost 15% of the 'Hump' airlift tonnage each month during 'Matterhorn'. Lieutenant General Albert C. Wedemeyer, who replaced Stilwell as the senior US commander in the China theatre, agreed with Chennault, and the two men were therefore happy to see the B-29 bombers leave China and India. Despite those objections, however, 'Matterhorn' did benefit the Allied effort. The development and use of the bases in China bolstered Chinese morale and, more importantly, 'Matterhorn' allowed the strategic bombing of Japan to begin six months earlier than would have been the case had the campaign been forced to await the availability of based in the Mariana islands groups. The 'Matterhorn' raids against the Japanese home islands also demonstrated the B-29’s effectiveness against Japanese fighters and anti-aircraft artillery, and the operations from the Mariana islands group also profited from the streamlined organisation and improved tactics developed on the Asian mainland.

Arnold retained personal command of the 20^th AAF to prevent the diversion of assets from the B-29 effort against Japan, particularly to Admiral Chester W. Nimitz, who had command over all the US efforts in the Central Pacific, with Hansell as chief-of-staff and later commander of the XXI Bomber Command. Tentative plans had initially called for an operational force of 48 B-29 groups with between 1,000 and 1,500 aircraft, to be deployed in four bomber commands: these were the XX Bomber Command of four groups in the India-China theatre, the XXI Bomber Command of 16 groups in the Mariana islands group, the XXII Bomber Command of 24 groups in the Philippine islands group and on Okinawa, and the XXIII Bomber Command of four groups in the Aleutian islands group.

The bases in China were part of the China Burma India Theater for administrative purposes, and the commander of the XX Bomber Command had no control over stations, bases, units, and personnel not directly assigned to him, and none over shipping and other logistical support essential to the operation. However the commander of the XX Bomber Command reported directly to the Joint Chiefs-of-Staff in Washington and, unlike other CBI Theater personnel, was not under South-East Asia Command operational control.