Complex sentence: More Merlins were made for the four-engined Avro Lancaster heavy bomber than for any other aircraft; however, the engine is most closely associated with the Spitfire, starting with the Spitfire's maiden flight in 1936.
Split into simpler sentences: More Merlins were made for the four-engined, heavy bomber Avro Lancaster than for any other aircraft. However, the Merlin engine is most closely associated with the Spitfire; starting with the Spitfire's maiden flight in 1936.