Agent US was in advanced development by the end of World War II. When the United States Air Force (USAF) wanted a biological warfare capability, the Chemical Corps offered Agent US in the M114 bomblet, based on the four-pound bursting bomblet developed for spreading anthrax during World War II. Though the capability was developed, operational testing indicated the weapon was less than desirable, and the USAF designed it as an interim capability until it could eventually be replaced by a more effective biological weapon.