The X-1 is a sophisticated nuclear-powered jet airplane, originally built by Jonas Venture during the heyday of Venture Industries. It is currently the property of Rusty Venture, who uses it as his primary mode of transport. Brock Samson, H.E.L.P.eR, Kano, and Rusty are evidently the only individuals qualified to fly it, as no one else has ever been seen in the pilot's seat. It is powered by an onboard nuclear reactor and is capable of both supersonic and vertical flight, allowing it to land and take off from almost anywhere.
The X-1's name might be a reference to the "Cygnus X-1" songs which are featured on the Rush albums "A Farewell To Kings" and "Hemispheres", or it might simply be a reference to the US Air Force's use of "X" as the prefix for "experimental" aircraft.
The X-1's external design is modeled on the XB-70 Valkyrie, a futuristic, supersonic bomber from the 1960s that cost $1.5 billion to design but eventually became obsolete in the face of advancing technology and was scrapped without ever being deployed or the Concorde and the Tupolev Tu-144, both 70s supersonic commercial airliner notable for their square jet engines and delta wings. The Valkyrie's Space-Age design, massive price tag and brilliant white paint job made it a memorable symbol of the grandiose overoptimism of early Cold War military projects (including the effort to build a nuclear-powered aircraft, another expensive failure). The X-1's continued existence (apparently in excellent shape) thus serves as a constant reminder of the shadow cast by Jonas' historical success and technological superiority over his ever-struggling son, who at one point is forced to dress the 30-year-old jet up as an "experimental stealth bomber" in an effort to drum up new military business from General Manhowers and avoid having to sell the Venture Compound to pay his bills.
The X-1 was stolen by Jonas Venture Jr. in his quest to assume his father's mantle from Rusty but returned upon cessation of hostilities between the two. Jonas Jr has since built his own plane (which he pilots himself), the "X-X-1", which is identical in appearance to the X-1.
At some point of time during the series the X-1 is upgraded. Notable changes are the decal on the exterior (added Venture Industries logo) and a sleeker interior.