I'd put in a vote for the XB-70. Mach 3 bomber, half a million pounds, stainless steel construction, variable geometry - all in the early 1960's. It also looks awesome...
Not quite a one-off since they built two, but only one survives today.

DH106

I think you'll find the word is pedantry. (See what I did there!!)

22 built? Unique design.

Not biting :)

https://en.wikipedia.org/wiki/Rutan_Boomerang

No apology required....just one of those terms that seem to stand out, goes with the list of aviation related spellings/misspellings: hangar / hanger & Gipsy / gypsy. OK Mike...back onto your perch !!!!

Oh, you can add the Pilatus P4 to the list.... It morphed into the rather better known Pilatus PC6 Porter, later Turbo Porter.

I flew the prototype at Farnborough with Angus McVitie; landed it too!!

I read an article a long, long time ago that explained the thought process that led Burt Rutan to come up with this configuration. I think it started with a generic Beechcraft Baron and then discussed the different issues inherent in that design (or any typical light twin) and how the Boomerang configuration solved that. I wish I could find that article.

The Westland Wendover would appear to meet the criteria....never heard of it until fairly recently, so looked it up. ...of the first two words that sprung to mind, only me can be repeated.

The Wendover looks like the result of a Lysander accidentally ending up on a Lancaster production line! :sad:

Might have been in Flying as I think I recall it too. Possibly penned by Peter Garrison. They are all on line, see if I can find it later.

I could mention the Vickers Windsor.
A 4 engined bomber with Geodetic construction powered by Merlins; three built; assembled at Foxwarren near Brooklands but then disassembled and moved to Farnborough for test flying.
Had to have 4 mainwheels because the wings drooped when on the ground; notable for its pressurised crew compartments and remote controlled guns called 'barbettes' at the rear of each outboard nacelle. First one flew Oct 1943, followed by the second in Feb 1944 and third in Jul 1944. Development curtailed as the Lancaster was in full production and the end of the war was in sight.
Then there was the CMC Leopard lightweight bizjet. 2 built with twin jet engines and designed to carry a pilot and 3 passengers. Prototype first flew in 1988 followed by the second with more powerful engines in early 1997.
Project cancelled on the death of the designer, Ian Chichester Miles, in 2009.

The Hunting H126, jet powered and could dawdle along at 28 knots.
Two were ordered but only one completed and flown.

My first visit to RAE Bedford in 1974, I saw the '126 departing on the back of a lorry.
There was also the HP115 'slim' delta designed for swept wing research. Used piston Provost undercarriage legs, one of which decided to collapse on landing one day at Bedford; the runway controller (temporarily on detachment from Farnborough) told me he watched as the aircraft described a circle around his vantage point.

Truly "unique" as I believe only one example built was the Douglas DC4E.

Garrison on the Rutan Boomerang :

https://books.google.co.uk/books?id=...merang&f=false

another singular example would be the Miles M.100 Student

Surely there is only one 'unique' aircraft - The Wright Flyer. Everything else is a descendant.

You caught my attention! Sorry for the thread drift, but was it good at what it was designed for, to replace helicopters in low-level/low-speed observation tasks?

What really is unique here is the hilarious misuse of the simple word "unique".