Frontier Group Holdings Inc. agreed to buy Spirit Airlines Inc. for $2.9 billion in cash and stock in a deal that would create a discount airline juggernaut.

Ultralow-cost airlines, designed around cutting costs and fares by offering a more basic flying experience, have upended the airline industry in recent years. The deal announced Monday would bring together two of the biggest such airlines, which the carriers said would allow them to compete more aggressively against larger rivals.

“This transaction is centered around creating an aggressive ultralow fare competitor to serve our guests even better, expand career opportunities for our team members and increase competitive pressure, resulting in more consumer-friendly fares for the flying public,” Spirit’s President and Chief Executive Ted Christie said.

The companies valued the deal at $6.6 billion, including the assumption of net debt and operating leases.