"The Flying Car of the Future, Today, is still pulling a bit to the right."
Though its only physical manifestation existed in a dream, the vehicle was somehow recorded on video as shown in "Phineas' Birthday Clip-O-Rama!". This may mean that they actually built the car at some point.
The car had an auto-parking feature enabling it to guide itself to a safe landing when unmanned. However, there were a few design flaws with the car, the most notable being its tendency to veer to the right.
Phineas and Ferb made a landing tower for it as well, which fell over and crashed down upon the house because of the absence of a substantial bolt in its structure. It was for this reason they were sent to reformatory school.
Rescuing Phineas and Ferb
Candace piloted the car later, taking Jeremy with her intending to break her brothers out of the reform school, but as she approached the destination, the vehicle struck a tree and careened out of control before hitting the ground and coming to rest at the edge of a ravine. The car was destroyed when it fell into the ravine shortly thereafter.
When Candace grabs Phineas and Ferb's toolbox from the car, she closes it's door it and the car falls into the ravine.