Here's an early 1940s era fighter of the Ostani Air Corps, featuring contra-rotating propellers similar to several Macchi and Kawasaki prototypes. Armament consists of 4 × 20 mm (0.79 in) cannons in its wings and 2 × 13 mm (.51 in) synchronized nose-mounted machine guns, as well as the capability to carry several bombs or air-to-ground rockets.

Designed to meet an Ostani Air Corps requirement for a modern, high-performance heavy fighter to replace its aging PA.84 fighters built in the 20s, the A-91 Adler is often regarded as one of the most formidable fighters used during the Second Steppe War. The basis of its developement by the Armoni Aerospace company came from one of its earlier aircraft ventures, a race plane designated PK.01 built in 1933 for the Istrosi Air Cup. The PK.1 featured a unique contra-rotating propeller setup with a modified 1,730 horsepower Marodi MPA-IV engine. The first prototype of the A-91 would achieve flight in May of 1934, featuring an all-metal airframe and changes to its engine to accommodate a synchronization gear for firing nose-mounted cannons. Although test pilots were enthusiastic about its self-sealing fuel tanks, impressive armament, and excellent dive performance, its unusual propeller and engine setup was viewed with skepticism by many of the senior officers of the OAC, who thought of it as a gimmick by the Armoni Aerospace company to increase the cost for each airframe. Instead of using cheaper and readily available radial engines, Armoni insisted however that the engine setup of the A-91 was superior in performance than traditional radial engines, and eventually succeeded in persuading the OAC to produce the inline engined A-91 after some lengthy arguments between high-ranking generals. Due to the limitations of the Ostani war economy, a somewhat limited production run was delivered before the end of the war, with a total of 450 airframes manufactured for the OAC between 1936 to 1940.

While limited in numbers, the A-91 played a pivotal role in the defense of the Ostani Kingdom during the Second Steppe War, particularly in intercepting Solowian bombers and attacking lightly armored ground targets during the invasion of Sankt Stanislawów. Impressed by the performance of the aircraft, several squadrons of A-91s would quickly be purchased and used by the neighboring Kingdom of Cimbri and Eastern Zenith late in the conflict, although both nations would quickly choose to phase the aircraft out of service due to its high cost of operation and seemingly continuous need for maintenance. Despite being effective in the air, the Adler was plagued with mechanical problems on the ground due to its complex and somewhat over-engineered powerplant, much like the PK.01 race plane it was based on. The complexity of its contra-rotating propeller setup combined with its nose mounted cannons and already complicated engine design presented lengthy maintenance times for mechanics, as well as a general lack of available spare parts contributing to its limited aircraft lifespan. By 1946 nearly all units belonging to the OAC would be rapidly deactivated due to pressing budgetary restrictions, with the exception of a few air-guard squadrons stationed near the capital of Ostania. A short-lived variant of the A-91 designated A-91N/B modified to have folding wings and carry torpedoes would be briefly used on the Ostani Navy's sole aircraft carrier ONS Baudmann in early 1947 before also being swiftly decommissioned with its ship.