The XB-70.. Oh, dear Lord, where do I start ? SO much of that bird was untested stuff, they built 2 examples, the latter, more advanced example crashed, killing everybody aboard... They were literally worth more than if they'd made 'em outa gold.. They were to replace then then still New B-52...Possibly even still, the most advanced aircraft that ever flew... The changing of technologies in 1964, doomed them, hard to conjecture what might have happened should they'd gone into production. Wags & smartasses said it looked like a banana towing an orange crate..