I recently read an article on the BAC TSR-2, a British strike and reconnaissance aircraft that was cancelled in the 1960’s despite being an advanced design and having flying prototypes.
What cancelled military weapon project do you wish was never cancelled?
I was going to restrict this question to Cold War military aircraft as that’s my personal area of interest but figured people might prefer a broader scope.
B-70. So what it had the radar cross section of a barn–all it had to do is overfly a Red base at treetop level at Mach 3+…
Just thought of another good one–Project Thor.
Nothing says loving like a tungsten rod arriving from orbit.
Rods from God.
The IAI Lavi, of course.
A homegrown Israeli fighter jet that was supposed to take the place of the F-16, it was by all account an excellent plane that was cancelled - narrowly - in 1987 for economic/political reasons. I get why they did it, but still, it would have been nice to have our own fighter.
Was the B-70 capable of high speed at low level though? I was under the impression it was purely a high-altitude airplane, for its operating envelope anyway?
Interesting, I’d heard the name before but not much about it.
The Air Force’s Dyna-Soar.
The M8 “Buford” Armored Gun System
It was designed to replace the aging, and troubled, Sheridan as the air droppable light tank for the 82nd Airborne Division. It got through design and testing. It got formally approved for purchase, type classified and named after John Buford of Gettysburg fame. Then they cut it for budget reasons. The Sheridan instead got “replaced” by HMMWVs mounting TOW anti-tank missiles.
It may still be fielded though. It came up in the initial briefing slides I saw laying out the requirements and possible off the shelf systems for what became the Stryker Brigades. The M8 only fit the light tank role in that Brigade organization and one of the goals was maximizing common automotive systems. Unsurprisingly it wasn’t picked. BAE has put an upgraded M8 into competition with two other systems for the Mobile Protected Firepower procurement program. The program is very similar to what the Buford was designed for. A decision is expected early next year. Buford may get his namesake.
Project Orion. Project Orion (nuclear propulsion) - Wikipedia
Though I don’t know how military the study was. And the environmental after-effects would have been fearsome. But we’d have our Moon and Mars bases by now, if we wanted to, if it hadn’t been cancelled. Maybe Solar Powersats and monstrously sized space telescopes too?
The A-10B Thunderbolt II. The A-10A was supposed to only be the first generation of this plane. The follow-up design, which made it all the way through prototype flights, had two seats and a much more sophisticated electronic package.
Even had its own theme song:
Me and my arrow
Straighter than narrow
Wherever we go, everyone knows
It’s me and my arrow…
Super battleships that would have been built if both axis and allied powers continued warring strong into the early '50s. The super Yamato would have six 20-inch guns. The german H44 with eight 20-inchers. The US Montana class with twelve 16-inchers might have been supersceded by ones with at least 18.1 guns.
Maybe not in the vein of what the OP means, but it is a shame the YF-23 didn’t win the ATF. The RAH-66 Comanche was a nice-looking thing too.
And AIUI Crusader would have been a lot better than the Paladin.
ha! the first thing I thought of in response to just your title was indeed the TSR-2.
I’ve seen the realm thing at Duxford and it is indeed a badass piece of kit. Very fresh-looking for a 60 year old design. At least it flew though.
The Northrop YB-49. Developed shortly after WWII. It was an early flying wing concept. Many were built before being passed over for the B-36. Used only once, to drop an atomic bomb on the Martian machines in the original “War of the Worlds.”
Project Aurora. Or maybe it was completed after all?
Other interesting, if potentially more destructive, cancelled projects:
Project Sipapu, plans to put a particle beam weapon into orbit (I found an oblique reference here)
And of course the grand-daddy of them all, the project with the most potential to find us a new home (because it destroyed our current home!): ORION
Reagan’s “Star Wars” anti-missile defense system, though an actual Death Star would have been remarkably cool! 
The 1000-ton tank Ratte from WW2 Germany. That would have shortened the war by being a resource sink in addition to being a completely cool tank. Oddly enough I think it would have been more useful on the Soviet side due to their deep battle strategy: having 1 or 2 of these operating at several different places on the battlefield would have created some inevitable breakthroughs, and they could also be concentrated to defend any German breakthroughs. Germans concentrating them to achieve an offensive breakthrough just doesn’t seem as useful to me. But that would have been moot anyway since both sides had enough air power to render them useless after the first day of good weather.
I’ll always have a soft spot for the Crusader artillery.
I always wanted to see the RAH-66 Comanche fielded. There really hasn’t been a true scout helicopter for decades.