Mario Kart 7's development became an "act of emergency" when Nintendo realised they would struggle to meet the end of 2011 deadline.
Speaking in the Japanese edition of Iwata Asks, via NeoGAF, producer Hideki Konno revealed development began with an eight person team that desperately needed help.
Retro Studios was originally only going to assist with the Donkey Kong designs, but ended up revamping all 16 classic courses, while Nintendo developed new tracks.
The staff shortage arose when Nintendo EAD were forced to prioritise Nintendogs + Cats for the 3DS launch, neglecting future titles including Mario Kart.
Mario Kart 7 will be released in Australia next Saturday, December 3.