X-4 Bantam

One of the more notable features of the X-4 Bantam, built by Northrop, was its semi-tailless design. That is, the tail section lacked horizontal stabilizers, so researchers could test a theory that those components were a key factor in stability problems at transonic speeds up to about Mach 0.9.

In the end, it was more like the opposite. "The X-4's primary importance involved proving a negative, in that a swept-wing semi-tailless design was not suitable for speeds near Mach 1. Aircraft designers were thus able to avoid this dead end," NASA said. Eventually, computer-driven fly-by-wire systems did allow for semi-tailless designs in production aircraft, such as the F-117 Stealth Fighter.

The two X-4 aircraft made a total of about 90 flights from 1948 to 1953.