http://en.wikipedia.org/wiki/B61_nuclear_bomb
“In May 2010 the National Nuclear Security Administration asked Congress for $40 million to redesign the bomb to enable the Lockheed Martin F-35 Lightning II to carry the weapon internally by 2017.[21] This version is designated Mod 12.[22] The four hundred B61-12 bombs will be used by both tactical aircraft (such as the F-35) and strategic aircraft (such as the B-2) and the Tail Subassembly (TSA) will give them Joint Direct Attack Munition levels of accuracy, allowing the fifty kiloton warhead to have strategic effects from all carrying aircraft.[23] However, refitting the 400 weapons is now expected to cost over $10 billion.[24] The B61 Mod 12 tail assembly contract was awarded to Boeing on November 27, 2012 for $178 million.[25] Boeing will use their experience with the Joint Direct Attack Munition to yield JDAM equivalent accuracy in a nuclear bomb.[26] This contract is only the first part of the billion dollar expense of producing and applying the tail kits, over and above the $10 billion cost to refurbish the warheads.[27] JDAM tail kits for conventional bombs of similar size and weight typically cost approximately $25,000 per unit,[28] about 1% of the B61 upgrade. The B61-12 uses an internal guidance system and can glide to its target. The bomb has four selectable yields: 0.3; 5; 10; and 50 kilotons.[29]”