Milton Wood Raymond, aka "Milt" or "MWR", wanted to build a faired (streamlined) bicycle that would be safe and have as little frontal and side profile as possilbe, to get the best forwad speed and minimize tipping in side gusts.
One problem with conventional recumbents is a large wheel does not fit readily between the riders knees, leading to compromises such as a small wheel, high rider position, longer wheelbase, and so on.
Milt built "X-1" to experiment with facing the rider backwards.
X-1 used a unicycle with the seat removed and laid horizontal, connected by a large-diameter steel tube (roughly 70mm OD and 0.9mm wall thickness) connected to the front end of a Raleigh "Chopper" frame. The frame was as low to the ground as possible and the riders recumbent (seated with their legs horizontal) to make balance be the worst case possible.
The first configuration used one rider facing and pedaling backwards, while a forward rider steered. This demonstrated that even at very low speeds (roughly 2 km/h) balance was easy to achieve, albeit without the ability to ride no-hands.
The second and final configuration removed the forward rider part of the frame and added under-seat linkage steering for a rider facing backwards.
In the second configuration, forward vision was initially none, then through a mirror mounted to the frame, then a mirror mounted on bearings on a vertical axis and lightly sprung to center so minor balance motions would not cause the mirror to steer, but large motions would steer the mirror.
Milt envisioned the mirrors could someday be replaced by electronic cameras and displays, but as X-1 was built in the early 1980's, cheap and lightweight video electronics were not available.
X-1 was successful as far as it went: it demonstrated it was easy "enough" to learn to ride a backwards-facing bicycle. However, poor forward visibility led Milt to work on X-2, a forward-facing design with two-wheel steering.