Nevertheless, during the 1920s, the US military, in particular, procured specialized "Attack" aircraft and formed dedicated units, that were trained primarily for that role. The US Army Engineering Division became involved in designing ground attack aircraft. The 1920 Boeing GA-1 was an armoured twin-engine triplane for ground strafing with eight machine guns and about a ton of armour plate, and the 1922 Aeromarine PG-1 was a combined pursuit (fighter) and ground attack design with a 37mm gun. The United States Marine Corps Aviation applied close air support tactics in the Banana Wars. While they did not pioneer dive bombing tactics, Marine aviators were the first to include it in their doctrine during the United States occupation of Haiti and Nicaragua. The United States Army Air Corps was notable for its creation of a separate "A-" designation for attack types, distinct from and alongside "B-" for bomber types and "P-" for pursuit (later replaced by "F-" for fighter) aircraft. The first designated attack type to be operational with the USAAC was the Curtiss A-2 Falcon. Nevertheless, such aircraft, including the A-2's replacement, the Curtiss A-12 Shrike, were unarmored and highly vulnerable to AA fire.