Regarding early jets, the Italians flew the firts jet plane (Caproni). However, it was a hybrid-the compressor was powered by a piston engine.
As far as the ME-262 goes, yes, it was fast-but the engines life was measured in hours. Too late and too little.
I saw a German ME-262 “Komet” at the 8th Airforce Museum-it was the first (and only) rocket powered fighter. I think it killed more of its pilots than allied airmen.
The SR-71?
That would be the ME-163 actually. Very interesting aircraft, but not one I’d care to fly.
What if you could put a small, modern turbojet in it?
The performance would be about the same. The Me-163s top speed was limited by aerodynamics not engine power, as the plane aproached the speed of sound it would suffer from Mach tuck which would force the nose down. From this page:
“On the Me 163, the combination of the aft shift in aerodynamic center and shock stall led to a dangerous condition known as “Mach tuck.” If the Mach number exceeded approximately 0.85, the airplane would begin to nose down on its own. The pilot would naturally react by pulling on the stick and deflecting the elevons upward. This would cause a shock wave to form on the underside of the wing at the elevon hinge line. The elevons would shock stall and be unable to bring the nose up, causing the airplane to pitch over into an ever-steepening dive. The only hope for recovery was to wait until the airplane had dived to a lower altitude where the speed of sound is higher, thus reducing Mach number, and the elevons would regain effectiveness.”
Of course the biggest advantage of using a modern jet is that jet fuel doesn’t *dissolve *the pilot after a bad landing.
I wouldn’t want to be anywhere near the damn thing. The fuel and oxidizer spontaneously reacted on contact. It didn’t just kill the pilots, it could kill you just trying to put gas in it.
I’m not sure if I’d classify it as a fighter, though. It’s job was to go after allied bomber formations. It might be more accurate to call it an interceptor.
I’d call a bad idea 
Why, yes, yes he does – and it shames me that I immediately knew which “Alex” he was talking about. :o
Back to the OP, possibly ironclad warships might count. Kind of stretch, as they weren’t so much an incredible innovation, but rather something everybody was thinking about, but didn’t know how they’d actually work in practice. That they were a common idea is proved by the amazing fact that the first two ironclad ships-- of very different design and by opposite sides in the war – first went into action within a day of each other.
I’m not sure ironclads count. They weren’t exactly secret.
Also, in my case at least, US hisytory teaching tends to gloss over the achievements of other nations, and the French Gloire (and her sister ships) and the British Warrior preceded the Virginia and the Monitor. They may not have looked like ironclads, because they were covered with wood, but they were functional ironclads, with steam-driven screws and all (and sails as well – people used to hedge their bets).
The Warrior still floats.
I’d say the Millatreuse qualifies as a secret weapon that turned out to be real. Too bad they employed it poorly when they actually put it into action.
As far as Secret Weapons go, I’m still impressed by the Dam Buster bombs, which actually worked as advertised (and not as I, at least, would naively expect)
http://www.dambusters.org.uk/
But I don’t think it fits the OP’s criterion as “legendary” and much-talked-about. These came effectively out of nowhere. And that, I think, is the case of most secret weapons.
Actually, reading about the history of Secret Weapons (especially in WWII), the sad fact is that most of them didn’t really work at all. Explosive-powered Panjandrums, giant plane-torchers, and other off-the-wall weapons didn’t realy pan out. The Dam Buster bombs, the atomic bombs, and things like Radar were happy exceptions.
What they didn’t have th internet in WWII? Yeah, right. Next you’ll tell us that Churchill didn’t use his cellphone to text Roosevelt every day from his bathtub.
You rang?