while certainly not secret for the whole war, you could talk about the German ME-262. It was a revolutionary German jet fighter.
"The
Messerschmitt Me 262 Schwalbe or "swallow" was the first operational
jet powered fighter. It was mass-produced in
World War II
and saw action from late 1944 in bomber/reconnaissance and
fighter/interceptor roles. German pilots nicknamed it the Turbo, while
to the Allies it was the Stormbird. Although the Me 262 had a
negligible impact on the course of the warshooting down an estimated
150 Allied aircraft for the loss of 100 Me 262s, the majority of
aircraft grounded for lack of fuelthe jet was both well-known and
highly influential on post-war aircraft development."
http://en.wikipedia.org/wiki/Messerschmitt_Me_262