CBS and the CW released an official poster for The Flash Supergirl crossover in Supergirl‘s March 28 episode (Digital Spy).
The first race between the Flash and Superman was in the pages of Superman #199 from 1967. You can see the television series understands its history.
“‘Worlds Finest’ – Kara gains a new ally when the lightning-fast superhero The Flash (Grant Gustin) suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire, in exchange for her help in finding a way to return him home.”
