When the Douglas A3D Skywarrior first entered fleet service, its sole job was nuclear weapon delivery. Though crews were trained for conventional weapons delivery, some aircraft would be placed on nuclear alert on the deck. The Forrestal-class carriers carried a full 12-plane squadron of A3Ds, the smaller Midway-class carried nine A3Ds, and the smaller Essex class carriers had a three plane detachment of Skywarriors. Regardless of the size of the Skywarrior unit, two A3Ds were always on the flight deck fully armed with a nuclear weapon and guarded by a Marine unit.
The "ready" aircraft and alert crews were on 15 minute alert to launch and deliver its nuclear payload.
Source: Strike from the Sea: U.S. Navy Attack Aircraft from Skyraider to Super Hornet, 1948-Present by Tommy H. Thomason. Specialty Press, 2009, p83.