In response to the feedback from the first prototype's testing, several improvements and other modifications were made, some of which was introduced on the original prototype while more extensive changes were introduced upon the second prototype. This second prototype, known as "PZL.37/II", featured the adoption of twin-vertical stabilizers, a re-worked cockpit and an altered ventral gun position amongst other changes, functioned as a representative aircraft of the anticipated production bomber and performed its first flight during autumn 1936. Suitably impressed, the department accepted the aircraft for production, placing an initial order for 10 pre-production aircraft, as well as bestowing a name upon the bomber, "Łoś".