RTX's [RTX] Pratt & Whitney said that it has spent more than $1 billion in the last five years to boost engine production--an investment that may come in handy, as the Pentagon on Monday said that it had awarded the jet engine maker a more than $3.8 billion contract for the F135 engine for Lots 18 and 19 of the Lockheed Martin [LMT] F-35 fighter. The total amount for the F135s in Lot 18 and 19 is now more than…