As best as I can tell the USN got it's first monoplane in 1937 and it first flew in 1936. To my knowledge, this was the TBD Devastator. (There are several other candidates including the BT-1 and SB2U, but neither of these was much if any earlier. The F2A was almost certainly later.) In any case, no navy had much of a jump. The IJN introduced the A5M at about the same time as the Devastator, and I believe that was Japan's first naval monoplane. I'd actually be inclined to say that this difference is so small as to be irrelevant to the discussion.