The installation of a radial engine in the P.Z.L. P.7 diminished the forward view for the pilot that was achieved in the P.Z.L. P.1 with its narrower V-12 engine, and it was proposed to improve this situation by the introduction of a Bristol Mercury radial engine, which was of smaller diameter than the Jupiter that powered the P.Z.L. P.7a.
This version of the fighter was designated P.Z.L. P.11, but delay in delivery of a Mercury engine from Bristol resulted in the P.11/1 prototype being flown initially, in August 1931, with a 384kW Jupiter IX.ASb engine licence-built by Gnome-Rhone. It was not until December 1931 that the P.11/II was flown with a 395kW Bristol Mercury IV.A enclosed in a long-chord Townend ring. This prototype was later re-engined with a 373kW Gnome-Rhone 9K Mistral engine, with which powerplant it was exhibited at the 1932 Paris Salon de I'Aeronautique.
A third aircraft with a Mercury engine, the P.11/III, served as a pre-production prototype and, following satisfactory official testing, was approved for production for the Polish air force as the P. 11a. However, it was preceded on the production line by 50 Mistral-powered P.11b aircraft for Romania, all of them delivered by the summer of 1934. Production of the P.11a began with a batch of 30, these being similar to late-production P.11b aircraft, but differed by having the 386kW Skoda-built Mercury IV.S2 engine.
The major production variant was the P.11c which adopted more radical measures to improve the pilot's field of view, lowering the engine and resitting the pilot farther to the rear on a raised seat, and a number of other improvements were incorporated at the same time. Production of this version totalled 175, the first batch being powered by the 418kW Skoda-built Mercury V.S2, but the remainder by the P.Z.L.-built Mercury VI.S2. A version of the P.11c, powered by a licence-built 9K Mistral engine, was built under licence in Romania by I.A.R. under the designation P.11f, about 80 being produced during 1936-38.
Deliveries of the P.11c to Polish fighter squadrons were completed by the end of 1936, and at the outbreak of World War II 12 squadrons were equipped with the type, claiming the destruction of 126 Luftwaffe aircraft for the loss of 114 of their own number. When, in early 1939, it became clear that the planned P.Z.L. P.50 Jastrzab fighter was unlikely to materialise, efforts were made to provide the P.11c with greater capability by the installation of a 626kW licence-built Mercury VIlla engine and four-gun armament. A prototype was flown as the P.11g Kobuz and quantity production was initiated, but the German invasion of Poland, had started before any of these aircraft were delivered.
Engine: 1 x Bristol Mercury VI.S2, 481kW
Wingspan: 10.72 m / 35 ft 2 in
Length: 7.55 m / 25 ft 9 in
Height: 2.85 m / 9 ft 4 in
Wing area: 17.9 sq.m / 192.67 sq ft
Max take-off weight: 1630 kg / 3594 lb
Empty weight: 1147 kg / 2529 lb
Max. speed: 390 km/h / 242 mph
Ceiling: 8000 m / 26250 ft
Range: 700 km / 435 miles
Armament: 2 x 7.7mm machine-guns, bombs