The deal includes a mix of Airbus A319, A320, and A321 models, the company said.
The purchase agreement finalizes an October provisional deal for the aircraft, when Spirit picked European planes despite Washington imposing tariffs on them.
Depending on the number of each variant of the A320 single-aisle family chosen, the deal could be worth $11 billion to $12 billion at the most recent 2018 Airbus list prices, but industry sources say such deals typically involve discounts of at least 50%.
Washington has imposed 10% tariffs on some of the planes Airbus offers to U.S. carriers, as part of a long-running transatlantic trade dispute over aircraft subsidies.
Spirit currently operates an all-Airbus fleet of 140 jets.
Aircraft are typically ordered several years in advance, meaning any planes ordered now would only be covered by tariffs in the event of an extended transatlantic tariff war. Airbus jets assembled at a plant in Alabama are not currently included.