The B-29 Superfortress was a very heavy bomber built by Boeing for increased speed, range, and bomb load, with features including a pressurized cabin, an electronic fire control system, and remote-controlled gun turrets. First flown on September 21, 1942, and introduced to service on May 8, 1944, it served with the U.S. Army Air Forces, U.S. Air Force, and Royal Air Force; 3,970 were built between 1943 and 1946. The Sixth Bomb Group trained here and deployed to Tinian Island in December 1944, where its B-29s operated from North Field, the largest and most expensive single construction project of World War II and still the largest and busiest airfield ever operated by the United States Air Force. The Sixth flew the longest combat missions of the war when ports in Korea were mined on the night of 11 July 1945, and the B-29 Enola Gay, carrying the Sixth's Circle R tail insignia, flew the first atomic mission. Tinian was captured from Japan and declared secure on August 2, 1944, after the June 15, 1944 assault on Saipan, Tinian, and Guam that cost more than 16,000 American soldiers. Almost immediately afterward, more than 15,000 Seabees of the 6th and 107th Naval Construction Battalions began converting the old Japanese airfield into North Field, using crushed coral, building four airstrips designated A, B, C, and D, and establishing an asphalt plant to pave the runways and roads. The first Sixth Bomb Group airplanes landed on Tinian on January 18, 1945, less than four months after construction began.