About Atlas B
The Convair SM-65B Atlas, or Atlas B, also designated X-12 was a prototype of the Atlas missile. First flown on 19 July 1958, the Atlas B was the first version of the Atlas rocket to use the stage and a half design with an operational sustainer engine and jettisonable rocket booster section. Unlike later Atlas models, the Atlas B used explosive bolts to jettison the booster section.