<P> A crossover between the two series was announced in July 2014 , after Barry Allen had been introduced in Arrow 's second season ahead of the debut of The Flash . By the following September , the scripts for each episode had been written , with filming occurring shortly before the end of the month and into October 2014 . The crossover sees every main cast member of each series who appear at least in their own series , with additional actors and characters also reprising their Arrowverse roles . </P>
when did the flash first appear on arrow
second season