http://en.wikipedia.org/wiki/Nuclear_weapon
http://en.wikipedia.org/wiki/Nuclear_weapons_delivery

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Nuclear weapons delivery is the technology and systems used to place a nuclear weapon at the position of detonation, on or near its intended target. Several methods have been developed to carry out this task.
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Weapons delivery
Main article: Nuclear weapons delivery
The first nuclear weapons were gravity bombs, such as the "Fat Man" weapon dropped on Nagasaki, Japan. These weapons were very large and could only be delivered by a bomber aircraft

Nuclear weapons delivery—the technology and systems used to bring a nuclear weapon to its target—is an important aspect of nuclear weapons relating both to nuclear weapon design and nuclear strategy. Additionally, developing and maintaining delivery options is among the most resource-intensive aspects of nuclear weapons: according to one estimate, deployment of nuclear weapons accounted for 57% of the total financial resources spent by the United States in relation to nuclear weapons since 1940.[6]

Historically the first method of delivery, and the method used in the two nuclear weapons actually used in warfare, is as a gravity bomb, dropped from bomber aircraft. This method is usually the first developed by countries as it does not place many restrictions on the size of the weapon, and weapon miniaturization is something which requires considerable weapons design knowledge. It does, however, limit the range of attack, the response time to an impending attack, and the number of weapons which can be fielded at any given time. Additionally, specialized delivery systems are usually not necessary; especially with the advent of miniaturization, nuclear bombs can be delivered by both strategic bombers and tactical fighter-bombers, allowing an air force to use its current fleet with little or no modification. This method may still be considered the primary means of nuclear weapons delivery; the majority of U.S. nuclear warheads, for example, are represented in free-fall gravity bombs, namely the B61.[2]