During early 1936, the necessity of revising the design's structure delayed the construction of the partially-assembled first prototype. During May 1936, the prototype commenced ground testing and taxiing trials; however, on 15 June, while being rolled out for its intended first flight, an engine failure was sustained prior to the attempt, resulting in its delay. On 30 June 1936, the first "PZL.37/I" prototype, which was fitted with a single vertical stabilizer, performed its maiden flight successfully. In August 1936, the initial factory tests were completed, after which the first prototype participated in official airworthiness and service trials. These revealed a handful of shortcomings, including rear fuselage flutter, excessive vibration on the instrumentation panel, overheating of the cylinder heads, and criticism of the cockpit's lack of space, however, test pilots were typically highly impressed with the aircraft.