Republic Airways has firmed up an order for 100 Embraer 175s, first unveiled at the Farnborough air show in July.
Deliveries from the order, which has a list-price value of $4.69 billion, will begin in 2020.
The deal includes purchase rights for another 100 E175s, with conversion rights to the E175-E2.
"This order represents another significant advance in our long-standing partnership with Embraer, and it positions Republic to compete for the more than 300 regional aircraft we anticipate being up for bid during the next five years as existing flying agreements expire with our global codeshare partners," states Republic chief executive Bryan Bedford.
Republic's operating subsidiary Republic Airline is the world's largest E-Jet operator with an in-service fleet of 188 E170/175s.
The carrier is a regional partner to American Airlines, Delta Air Lines and United Airlines.