XB-70 Valkyrie

A 1960s coldwar supersonic strategic nuclear bomber, and holder of the record for the largest Experimental plane. It could fly at Mach 3.08 and achieve an altitude of 74,000 ft using it's 6 engines. When using the experimental fuel ethyl borane, one engine alone made more noise than any air-breathing engine in history. Development of the two prototypes was to cost around $1,500M USD, making them the most expensive two aircraft built to that date.