wuzak
Captain
Heinkel's first jet was far ahead of any other fighter and could have been deployed in 1940.
Doubtful.
He 178 first flight was in August 1939, it had a decent, at the time, top speed ~380mph, but range and endurance were short, it had no armour or armament. It was, basically, a research aircraft, like the (later) Gloster E.28/39.
Heinkel's 2nd attempt, the He 280, first flew in 1940, but only as a glider. It wouldn't fly under its own power until 1941.