Sentence:  North American [Aviation] (NAA) added six General Electric X279 engines to the large, flat <WSD>afterbody</WSD> and turned the "wedge" into a pair of two-dimensional variable-geometry air inlets to feed the engines, and it fleshed out the vexing problem of what to build the plane out of using the experience it had gained working on the Navaho missile's structure. 
Meaning of the word marked with <WSD> tags:  The afterpart of a vehicle.