Complex sentence: It was a jet-powered development of the piston-engined Northrop XB-35 and YB-35; in fact, the two YB-49s actually built were both converted YB-35 test aircraft.
Split into simpler sentences: It was a jet-powered development of the piston-engined Northrop XB-35 and YB-35. The two YB-49s actually built were both converted YB-35 test aircraft.