Complex sentence: The General Electric J31 was the first jet engine produced in quantity in the United States, essentially a production version of the prototype Whittle W.1 that had been sent to the US after the Tizard Mission successes.
Split into simpler sentences: The General Electric J31 was the first jet engine produced in quantity in the United States. The J31 was essentially a production version of the prototype Whittle W.1 that had been sent to the US after the Tizard Mission successes.