The air conditioner struggled to cool the crowd crammed into Billy Bob’s Texas to see Midland, one of country music’s unlikely success stories, play a sold-out summer show. But a rapid trade in Shiner and Dos Equis kept the fans chilled until the members of the Dripping Springs–based trio, generally considered too traditional for country radio stations and a little too slick for their Americana counterparts, swaggered onstage in their signature retro-rhinestone, cowboy-chic attire. After admitting to partying until 5 a.m. following a performance the night before, Mark Wystrach, Jess Carson, and Cameron Duddy forged ahead with a slew of classic-sounding, dance floor–ready honky-tonk tunes.One song, though, stood out from the rest: a now-iconic arpeggio was all it took to get the crowd to scream,…