The Curtiss Falcon series of attack aircraft used throughout the mid- to late-1920s and into the early 1930s were the last biplane attack aircraft used by the U.S. Army Air Corps. The Falcons were also the last attack aircraft developed as derivatives of existing observation aircraft. The development of attack aircraft paralleled the latest technologies being incorporated into latest pursuit aircraft and bomber of the early 1930s. The all-metal monoplane rapidly made all biplanes in service obsolete. Attack aircraft development retained a few established and outdated design standards also. The most obvious example of this was the use of external bracing wires for the wings. The early 1930s production types did not have retractable landing gear either -- it wasn't until 1935 and the Northrop A-17 that an Army attack aircraft in service had retractable landing gear.
The development of the monoplane attack aircraft began in late 1929 with the Atlantic-Fokker XA-7 and Curtiss XA-8. The Curtiss design, named Shrike by the company, formed the based of an entire series of variants. Like the Falcon attack aircraft and the Hawk pursuit aircraft, the Shrike models were very similar with the major difference being the installation of a new type of engine.
The mid-1930s saw increasingly more capable designs. The Northrop A-17 was the primary attack aircraft in the years leading up to World War II. Because of continuing budget constraints into the mid-1930s, several compromises had to be accepted in the interest of getting aircraft into service. In the case of the A-17, a smaller engine was substituted when stability problems appeared early in the test program. The engine replacement was cheaper and quicker than a redesign and modification of the tail assembly.
Where the early attack designs were based on single engine observation aircraft, the late 1930s saw the rise of the attack aircraft based on the light or medium twin-engine bomber design. The first aircraft of this type to reach production was the Douglas A-20 Havoc. The A-20 was produced throughout WWII in many different models.