The 2015 Ford Mustang has now been revealed but there were a lot of months when it was disguised and it was hard to see what it would actually look like. It was really well camouflaged most of the time and Ford were so proud of the job that they did that they even released a video to show how they managed to fool so many people.
The disguise on the car may have looked very crude but there was actually a design team that put together the foam, vinyl and plastic to hide the design of the car underneath. They wanted to make sure that the final look of the car was not revealed but it actually managed to make sure that a lot of people were interested in it. Ford have even said that within an hour of the car being put on a road there were spy shots appearing online. This was probably in June when the first official shots were published.
There is also a video of the car testing with a lot less camouflage which was likely to have been recorded back in August.