VC-10. Good aircraft, too few, too late. A good aircraft. Steamrolled by the 707 and DC-8
Convair 880. Too small, too fast, too expensive to operate in seat per mile. The market for a speed demon never developed. Steamrolled by the 707 and DC-8 ala VC-10
DC-10. A wide body competitor on the cheap. Never should have been built as it was. Needed seperation of various systems to avoid potential catastrophic failures
Douglas A3D (aka All Three Dead). Basically the same aircraft as the B-66 which was derived from it. The lack of ejection seats in a military aircraft was bordering on criminal.
Martin P6M Seamaster. Beautiful flying boat. Think of it as a flying boat version of the Handley Page Victor. It also suffered from some similar design issues that plagued the Victor primarily issues involving the T-tail. Never entered service. A concept that was dated before it left the drawing board.
Lockheed Tri-Star. A midsized widebody not done on the cheap. Just too expensive to seriously compete with the DC-10 and not enough capacity to take on the 747.
Martin 202/404. Intended as a replacement for the DC-3 (how many times has that been tried) as was the Convair CV-240/440 family. Both were equipped with possibly the best piston radial engine ever built. The Pratt & Whitney R-2800. Martin through poor decision making used a poor alloy choice leading to scructual cracks in the wing sparks. On top of that Martin went unpressurized limiting altitude (the Curtis CW-20/C-46 was in production ). By the time the Martin caught up they got buried by Convair.
The Beechcraft Starship. Too novel and too strange for the market. I'll give Beechcraft credit for trying though.
The P-63 Kingcobra. Redundant and un-needed. Its single contribution to the US war effort was as the Pinball version used to provide air gunner trainies a fighter aircraft target to shoot at during training with special .30 cal lead/bakelite ammunition that would shatter on impact. Two modified into L-39-2 swept wing test aircraft used to test swept wings at low and stall speed.
The primary problem most of these aircraft faced was a design or intended use set in a flawed idea.
One last one. The absolute worst airplane ever built. Two built, two crashing on their first flight both killing their pilots. The incomparable Christmas Bullet. Actually a good idea in terms of attempting to eliminate all of the struts and wire used on then current aircraft. If just lacked any serious foundation in engineering.