SAN ANTONIO — An investment firm has purchased a majority stake in Whataburger as the Texas-based fast-food company seeks to expand in an increasingly competitive market.

Whataburger announced Friday that the purchase by Chicago-based BDT Capital Partners should be finalized by the end of the summer and will be subject to regulatory overview. Terms of the deal were not disclosed.

BDT will provide long-term capital and guidance as Whataburger, which has its headquarters in San Antonio, competes at a time when convenience stores are offering more food options and eating into market share.

The firm often invests in family-owned companies.

The first Whataburger opened in 1950 as a small roadside burger stand in Corpus Christi. It now has nearly 700 restaurants in Texas and more than 150 others in southern states extending from Arizona to Florida.