The XXI Bomber Command was a unit of the Twentieth Air Force in the Mariana Islands for strategic bombing during World War II.

The Marianas chain of islands, consisting primarily of Saipan, Tinian, and Guam, were considered as being ideal bases from which to launch B-29 Superfortress operations against Japan.

[2] Serious planning for the movement of the XXI Bomber Command's B-29s from their Second Air Force training bases in Kansas to newly constructed combat airfields on Saipan, Tinian and Guam began in April 1944.

After several weeks of heavy fighting, during which over 3000 American and 24,000 Japanese people died, the island was finally declared secure on 9 July.

The Marine Corps and the United States Army 77th Infantry Division's 305th Regimental Combat Team landed on Guam.

This was later renamed Isley Field, after Navy Commander Robert H. Isely (unfortunately his name was misspelled and the incorrect version stuck).

The XXI Bomber Command was assigned the mission of destroying the aircraft industry of Japan in a series of high-altitude, daylight precision attacks.

[2][3] In late October and early November 1944, a series of tactical raids were carried out as training exercises for the crews.

The submarine pens at Truk were the target, and four Superfortresses had to abort because of the usual engine problems, and combat formations were scrappy.

[2][3] Aware that there was now a new threat, Japanese aircraft based on Iwo Jima staged a low-level raid on Isley Field on 2 November, damaging several B-29s on the ground.

As in the XX Bomber Command Operation Matterhorn bombing campaign from India, the B-29s were in danger of being dissipated in tactical missions and even these were not all that successful.

[2][3] In order to properly plan missions to Japan, up-to-date reconnaissance photos of the proposed targets were needed.

On 1 November 1944, two days after arriving on Saipan, A 3d Photographic Reconnaissance Squadron F-13A Superfortress (photo reconnaissance-configured B-29) took off bound for Tokyo.

Also, for the first time, the B-29 encountered the Jet stream, which was a high-speed wind coming out of the west at speeds as high as 200 mph at precisely the altitudes at which the bombers were operating.

Construction Battalions extended and transformed the former Japanese fields to accommodate B-29s and the first AAF units moved in during the beginning of March.

[2][3] The high-altitude bombing raids on Japan carried out by the command were not causing a large amount of damage to the targets, primarily due to the jet stream winds over the islands.

Also General Hap Arnold and the AAF command staff which were aware of the Manhattan Project, and planned on using the B-29 to drop the Atomic Bombs, were concerned that the B-29 would be unable to carry out that highly secret mission.

In addition, LeMay had concluded that the effects of the jet stream, cloud cover, and high operating altitudes were to blame for the failure of the B-29 raids to do any significant damage to the Japanese war industry.

LeMay suggested that high-altitude, daylight attacks be phased out and replaced by low-altitude, high-intensity incendiary raids at nighttime.

With these new tactics, a total of 302 B-29s participated in the Operation Meetinghouse raid on Tokyo on the night of 9–10 March, with 279 arriving over the target.

The individual fires caused by the bombs joined to create a general conflagration, which would have been classified as a firestorm but for prevailing winds gusting at 17 to 28 mph (27 to 45 km/h).

[2][3] A month later, on 12 April, the 314th Bombardment Wing's remaining two groups, the 39th and 330th, joined in the attack on the Hodagaya Chemical Works in Koriyama.

In May, the 58th Bombardment Wing completed its move from India to Tinian, adding four more groups to the XXI Bomber Command.

On 7 April 153 B-29s struck the Mitsubishi Heavy Industries aircraft-engine complex at Nagoya, destroying about 90 percent of that facility.

The antenna swept a 60-degree arc along the flight path of the plane, and a higher frequency (X-band) signal gave a much-improved radarscope picture.

In late June, B-29 crews felt sufficiently confident that they began to drop leaflets warning the population of forthcoming attacks, followed three days later by a raid in which the specified urban area was devastated by mass carpet bombing.