The Mark XXXI Armor (codenamed Piston) was Tony Stark's thirty-first Iron Man suit and was a mix of the Mark XXI and the Mark XXX, making it both fast and power efficient. It was a part of the Iron Legion. It was destroyed during the Battle on the Norco.
The Piston Armor was called along with the rest of the Iron Legion to help Tony Stark and James Rhodes during their fight against Aldrich Killian and his Extremis Soldiers. The Piston was among one of the first Armor Suits to fight against the Extremis soldiers and after knocking one out of the four enemies, the Mark XXXI was destroyed when surrounded and pounded by three remaining ones, while Piston managed to kill the last one by ramming him to the ground creating a devastating explosion that rendered a platform on the docks unstable, which needed to be supported by the Mark XXXVIII armor 
The Mark XXXI can fly at supersonic speeds and not have to recharge for long periods of time.
| Mark I • Mark II • Mark III • Mark IV • Mark V • Mark VI • Mark VII|
Mark VIII • Mark IX • Mark X • Mark XI • Mark XII • Mark XIII • Mark XIV • Mark XV • Mark XVI • Mark XVII •
Mark XVIII • Mark XIX • Mark XX • Mark XXI • Mark XXII • Mark XXIII • Mark XXIV • Mark XXV • Mark XXVI •
Mark XXVII • Mark XXVIII • Mark XXIX • Mark XXX • Mark XXXI • Mark XXXII • Mark XXXIII • Mark XXXIV •
Mark XXXV • Mark XXXVI • Mark XXXVII • Mark XXXVIII • Mark XXXIX • Mark XL • Mark XLI
Mark XLII • Mark XLIII • Mark XLIV/Veronica • Mark XLV • Mark XLVI • Mark XLVII
|Mark I • Mark II/Iron Patriot • Mark III|
|Mark I • Mark II|