The network's plan was to originally have Barry Allen (Grant Gustin) appear in three Season 2 episodes of Arrow, Episode 8 ("The Scientist"), Episode 9 ("Three Ghosts") and Episode 20. The character will still appear in Episodes 8 and 9, which will serve as an origin story for Barry Allen, but instead of turning Episode 20 into a back door pilot, the network has opted for a traditional pilot.
The decision was made after network executives saw rough cuts of "The Scientist" and "Three Ghosts", which were well-received. By shooting a traditional pilot, it allows the creators to flesh out the superhero's story and world. The Flash is no longer a "full-blown spin-off," but it is still connected to the Arrow universe.
Here's a description of "The Scientist", airing Wednesday, December 4 on The CW.
""A seemingly impossible robbery at Queen Consolidated's Applied Sciences Division brings Central City police scientist Barry Allen (guest star Grant Gustin) to town. Citing a similar case back home, Barry offers to help Oliver and team with the investigation. Oliver senses there is more to Barry than meets the eye, but he's distracted by the similarities between this current case and something that happened on the island. Meanwhile, Felicity takes a liking to Barry, which doesn't go unnoticed by Oliver. Sin (guest star Bex Taylor-Klaus) asks Roy for help when a friend of hers goes missing. Roy is surprised when Thea not only encourages him to help, but joins the search. Unfortunately, Sin's friend is connected to Brother Blood (guest star Kevin Alejandro), and their search ultimately gets one of them seriously injured."
It isn't known if the currently untitled Episode 20 will make any reference to Barry Allen or not. The producers are expected to discuss their ideas for the episode with network executives this week. Greg Berlanti, Andrew Kreisberg and Geoff Johns are writing The Flash pilot script, with David Nutter set to direct.