U.S. officials say adding the B61-13 bomb to America's nuclear arsenal is key to holding certain underground and large area targets at risk. he first production example of the new B61-13 nuclear bomb has been completed roughly a year ahead of schedule, according to the U.S. Department of Energy. The B61-13 is based on the preceding B61-12, but offers a higher maximum yield that U.S. officials say offers important additional capacity to hold hardened targets, such as underground command and control facilities, and ones spread across physically larger areas, at risk.The U.S. military first announced plans to develop and field...