The de Havilland Comet which “inaugurated the jet age” for passenger travel in 1952 had really neat large, square windows. Travelers loved them.
The ones that survived, anyway.
Actually, it’s the desire for low fares that never gets obsolete.
You can fly acrosss the country or across the Pacific in amazing hotel-like comfort. Just plan to spend a few thousand dollars per hour for the privilege. If you only want to spend $100/hour, you can only have a small fraction of the space, amenities, luxurious surroundings, or pampering. And for $50/hour (ie ~$400 NYC-London), it gets even less attractive.
In the heyday of the steamship, Queen Mary & all that, steerage was a LOT less expensive than First Class. It was also a LOT less pleasant. The difference between the accomodations on an executive jet and on Brand X PackageVacationTours is equally stark. The good news is now you only have to put up with it for 6 hours, not 6 days.
The funny thing is how folks somehow expect a product designed to be as cheap as possible given current technology & input costs to not be nasty by current standards of civility.
I gotta go with the Boeing 2707-100.
I must say, in my humble opinion, it could have made the Concorde it’s bitch.
I have a soft spot for the Ryan Fireball.
As an Israeli, I have to mention the Lavi for pretty much the same reason.
Hell yes. It’s only within the last year that I’ve learned that the Bv141B wasn’t the weirdest of their asymmetrical flights of fancy.
I’ve a soft spot for the Kyushu Shinden myself. Unfortunately history doesn’t promise a dream future for a brilliantly-conceived and futuristic Japanese plane that maidens in the first week of August 1945. :smack:
How about the Boeing “Sonic Cruiser”. Probably just a smoke screen to divert attention from the more conventional B787, but a nice looking concept all the same.
Do gliders count, too?
If so, I’d like to introduce the Antonov KT .
Do fictional planes count?
Then I vote for the Stiles Skystreak from the movie “The Great Waldo Pepper”. I so wanted him to do the outside loop in it…
The Boeing Pelican ground-effect plane. At least I assume it didn’t make it, I haven’t heard about it in a while.
And be prepared to spend several times coach fares for even an incremental increase in leg or elbow room. It’s not a perfect market, Guy; it’s an oligopoly, and the fix has been in for decades.
Ah, but for a plane with real potential (unfulfilled), try the DOUGLAS XB-42 MIXMASTER!
A medium bomber, with tail-mounted, contra-rotating pusher propellers!
Top speed over 400mph!
4 2000lbs bombs!
Doubled as an attack aircraft!
How I wish the US had that at the start of WW2. 
The Pittw Sky Car.
Pitts Sky Car. Damn typos.
Can’t say this enough in answering the question of the demise of huge flying boats. Why would an airline continue to pay for their own exclusive dock/port facilities when they could now use government subsidized land facilities? Also, they no longer needed to look for sheltered, calm waterways to expand their routes. Large airstrips were virtually everywhere anyone wanted to go.
Lets not forget rotary winged aircraft. How about the AH-56 Ceyenne? it was way ahead of it’s time. It never got beyond the prototype stage because of problems with it’s advanced weapons systems and because Bell started saying “lookee here we built you a Cobra.” The Army decided to go with the proven technology of the Cobra which was basically a Huey in a narrow body. They then adapted the Cobra to fit the anti-Soviet/Fulda Gap mission that the Cheyenne was designed for. It wasn’t until the Apache came along that the US Army had a true Attack Helicopter that was designed for that role from the ground up.
I had a model of that when I was a kid.
If you think carefully about it, would you like to be in an airplane nicknamed “The Fireball”? 
I’d read that counterrotating propellers, although nifty, have always had enough problems that they’ve been of limited use.
How about the “GOBLIN” mini-fighter (it was launched and recovered from a B-52 bomber. Unfortunately, it was too small to do a whole lot. Does one survive?
How about the Convair XC-99?