Hey all, here for you is the B variant of the Litvinov PI-37;
Shortly after the PI-37A entered service, the SRAF issued a new requirement for a high altitude fighter and interceptor. The standard PI-37 was found unsuitable for this task as the radial engine had poor altitude performance. Movich, the primary supplier of aero-engines for the SRAF proposed modification to the airframe to allow for the Movich V-1350 inline engine (a development of the Hispano-Suiza 12Y proposed but rejected for the Korolev TB-31A.
Litvinov adapted the airframe and lengthened the nose to accommodate the new engine as well as to maintain balance given the lighter weight of the engine. The new engine offered superior performance at altitude than the radial and granted improved climb performance. In counter however the inline engine was more susceptible to damage at low altitude and had inferior engine response times. The new engine also reduced the armament to twin 20mm guns in the wings and a single 7.62mm gun in the port nose cowling, later replaced by a 12.7mm gun.
The new fighter dubbed the PI-37B saw more diverse service than its A variant abroad. The B series saw extensive service over China before and after the declaration of war against Japan. A few captured Chinese frames saw testing and evaluation by the Japanese against their own aircraft. Unlike the A series, the B never saw operations onboard carriers given the Navy's preference for more compact radials.