At a cost of ~3000 American and 24,000 Japanese lives, the Northern Marianas island of Saipan was taken in July 1944. With the subsequent seizure of Guam and Tinian in August, the U.S. now had ideal locations (~1500 miles from Japan) to construct airfields within the range of the new B-29 superfortress (~5000 miles). Additionally, these new airfields could be directly supplied by ship from the USA.
The 4-engine Boeing B-29 Superfortress was one of the largest aircraft flown during WWII. (Other behemoths included the German Messerschmitt Me-323 and Junkers Ju-390). With state-of-the art technology, the development and manufacture of the B-29 was the most expensive American weapons project of the war (far more expensive than the A-bomb Manhattan project).