Grant Gustin is set to appear as Barry Allen in the December 4th episode of Arrow, “The Scientist”, and early reports are indicating that the CW bigwigs are pleased with the results. As such the word is that the origin story will go ahead in its own pilot, with a mid-season debut planned if the pilot is picked up. Greg Berlanti, Andrew Kreisberg and Geoff Johns will still write the pilot, with David Nutter directing.
The character was originally planned to appear in episodes 8, 9 and 20, with the latter originally serving as a back-door pilot for a new series. The Flash, in his complete red costume, will now make his television debut in the new pilot at a later date. While this may not make it as close of a spin-off as the original plan, the character is still being introduced in Arrow, and the shows will always be linked as a result.
From Green Arrow TV, we have the first images of Gustin as Barry Allen below: