X-Planes: Experimental aircraft tested by US government bodies like NACA, (which later became NASA) and the US Air Force. There have been over fifty X-planes. Wikipedia has a full list, but here are some of the more notable ones:
X-1 was the first plane to exceed the speed of sound in level flight, with Chuck Yeager at the controls in 1946.
X-2 was first to exceed Mach 3 in 1952.
X-6 would have been the nuclear-powered bomber, if it had flown on reactor power. Instead, the program was cancelled in 1957.
X-13 was a VTOL jet that stood on its tail: 1955
X-14 was a VTOL jet that used vectored thrust instead: 1957
X-15 Exceeded Mach 6 and achieved spaceflight by reaching an altitude of more than 100 kilometers in 1963.
X-18 was a tiltwing: 1959
X-19 was a tandem tiltrotor (a tiltrotor and a quadrotor!): 1963
X-22 was a ducted fan quadrotor/tiltrotor in 1966.
X-49 was a gyrodyne, a helicopter with a (ducted pusher) propeller to go faster: 2007
Other X-planes had swing-wings, forward swept wings, no wings, rotors, lifting bodies, ramjets and scramjets. There are lots of different ways to get something into the air. Some of them have to be seen to be believed, and in a few cases, even a photograph leaves me wondering, “could that thing really fly?”
Tech Level: Mostly high to very high, although there’s an autogyro and a sailplane on the list. Genre-wise, it’s a mixed bag, with lots of candidates for Military SF and a few that best belong in Alternate History.
Appeared In: Spaceplanes have been a stock SF item since before they existed. Other machines on the X-plane list have probably not found a home in Spec Fic. Yet.
For Your Plot: I think some of the weirder classes of aircraft on this list could lend a nice twist to worldbuilding. Which pretty much sums up how I feel about Alternative Aviation in general.