Downey Jr, 45, will voice the role of the genius dog Mr. Peabody in the film, which was recently announced to staff at Dreamworks.
Mr. Peabody and his pet boy Sherman are known from the Rocky & Bullwinkle Show, where they travelled through time to find out interesting things about history's notable figures.
The role of naive boy Sherman has reportedly not yet been cast, according to EW.
The film's director Rob Minkoff said: 'Certainly for Boomers, they're in the sweet spot of our childhood animation memories, but beyond there it gets a little bit, well, not quite as familiar.'
Minkoff added: 'Mr. Peabody is this genetic anomaly. He does have brothers and sisters, all of them non-speaking, no super-smart dogs. He's an outcast, but has overcome it by being so great at so many things.'