Complex sentence: The XP-7 started life as the last Boeing Model 15 (PW-9D), serial 28-41; it was then adapted to mount the 600 hp Curtiss V-1570 Conqueror engine.
Split into simpler sentences: The XP-7 started life as the last Boeing Model 15 (PW-9D), serial 28-41. It was then adapted to mount the 600 hp Curtiss V-1570 Conqueror engine.