The P.Z.L. P.37 Los (Elk) was a modern twin engined medium bomber that was the most advanced aircraft in Polish service in 1939, but that was only produced in tiny numbers and equipped four squadrons during the German invasion of 1939.
P.Z.L. had first worked on a bomber design in the mid-1920s, producing the rather ungainly P.Z.L. 3. This was never built (although the French produced a very similar prototype as the Potez 41. Instead a licence built version of the Fokker F.VIIb-3 entered service in 1931, although only equipped three squadrons, with a total of 18 aircraft between them.
Work on a replacement for the F.VIIb-3 began in the early 1930s. Several designs were produced, including the Lublin R-XVIII biplane and a modified version of the P.Z.L. 30 passenger transport, but the successful design was produced by Jerzy Dabrowski. This was for a twin engined monoplane of all metal construction with a stressed skin. It was to be powered by Bristol Pegasus air cooled radial engines, and armed with twin machine guns and a 20mm dorsal cannon. P.Z.L. already had some experience of twin engined aircraft, having developed the P.Z.L. P.23 Karas but the P.37 was a much more modern design.
The Department of Aeronautics received the design in July 1934. It performed well in wing tunnel tests, and in October 1934 P.Z.L. was given permission to work on the detailed design. Dabrowski and the Department of Aeronautics decided to emphasis speed, so the heavy guns were replaced with single 7.7mm machine guns. Construction of a full scale wooden mock-up was approved on 14 April 1935, and work on three prototypes followed soon afterwards. The design also inspired the P.Z.L. P.38 Wilk fighter, but worries about its advanced design also meant that work continued on the P.Z.L. 30, which was built as the L.W.S. 4 Zubr. (Bison), a very ungainly looking aircraft that was virtually useless when it finally emerged.
Work on the first prototype, the P.37/I, was well underway by the end of 1935, but it was then delayed by a series of structural changes that were introduced in January 1936. In April static tests shows that the wings were too weak, so the prototype had to be modified once again. Ground tests began in May, and the prototype, powered by two 873hp Bristol Pegasus XII engines was finally ready for its maiden flight on 16 June 1936. However one of the engines failed during the engine test, and the maiden flight was delayed by two weeks. The P.37/I had an extensively glazed nose, a narrow fuselage, and a single tail.
The P.37/I was put through its factory trials in August 1936. Both factory and air force pilots flew it and were generally enthusiastic, although a few minor flaws did appear, including cracks in the exhaust pipes, overheating in the cylinder heads and an uncomfortable cockpit. An order for 30 P.37As, powered by the Pegasus XIIB, was placed and the type was named the Los (Elk).
The second prototype was produced during 1936 and made its maiden flight in the autumn. It had a new twin tail, 925hp Bristol Pegasus XX engines and improved positions for the pilot and radio operator. It also introduced a new undercarriage, with two small wheels side by side on each leg. These changes were adopted for the main production version, the P.37B.
The third prototype, P.37/III, was given two 970hp Gnome-Rhone 14N07 radial engines and was the development aircraft for any export orders. It made its maiden flight early in 1937. With its original engines it reached 280.5mph at 13,943ft. It was also later tested with 1,020hp Renault 14T engines, 1,030hp Fiat A.9- RC41 engines and 1,030hp Gnome-Rhone 14N21 engines.
A total of 180 aircraft were ordered by Poland, with the first 124 to be delivered by 1 April 1939. This first batch was made up of the 30 Los As, and batches of 50 and 44 Los Bs. Production began at P.Z.L.’s W.P.1 factory at Warsaw-Okecie in 1937. A much larger factory, W.P.2 at Mielec, was under construction, and in May 1939 the parts for 30 of the Los bombers were moved there to get production underway. The first aircraft from the new factory made their maiden flights in August 1939.
The thirty Pegasus XIIB powered Los As were split into two types – ten were completed with the original single tail as the Los A and twenty with the twin tail as the Los A bis. These aircraft were delivered to the 1st Air Regiment in the spring of 1938, but in June one aircraft lost its port wing. The type was grounded and the suspect part of the wing strengthened. However when it was cleared for flight once again problems developed with the twin tailed Los A bis, and eight were destroyed in accidents. It was eventually discovered that the rudders could lock in place if they were turned too far at high speeds. A simple fix solved the problem, but it had delayed the full service entry of the type. The Los B was also delayed, in this case by slow production of the Pegasus XX engine.
Despite these delays, about 90 aircraft had been delivered by 1 September 1939. Enough Los Bs had arrived to allow the Los A and A bis to be withdrawn to a conversion unit. In the autumn of 1938 Nos.211 and 212 Squadrons received the type, and in May 1939 Nos.216 and 217 Squadrons received their aircraft, bringing the front line strength up to 36 aircraft. However by this point the Los had fallen out of favour. It had been under attack for some time, partly because of the early problems and partly because some felt that bombers were a luxury that Poland couldn’t afford. Early in 1939 General Josef Zajac was promoted from Inspector of Anti-Aircraft Defences to head of the Air Force and he almost immediately attempted to cancel production of the P.37 with 105-110 aircraft built. However twenty were so far advanced that there was no point cancelling them.
The P.37 was more popular outside Poland. Two export versions were offered – the P.37C powered by two 970hp Gnome-Rhone 14N07s and the P.37D powered by two 1,030-1,050hp Gnome-Rhone 14N20/21s were offered. Several orders were placed. Yugoslavia ordered 20 P.37Cs. Bulgaria ordered 15 P.37Cs. Romania ordered P.37Ds. Turkey ordered ten P.37Ds, the parts to build another 15 and a licence to produce the type in Turkey. Several other countries were considering placing orders. However the German invasion ended all of these plans.
In the summer of 1939 the four P.37 Los squadrons joined with five P.23 Karas squadrons to form the Bomber Brigade, which was directly under the command of the Commander-in-Chief of the Polish Armed Forces. However when the Germans invaded it took several days to decide how to use the heavier P.37s. Plans for a raid on Konigsberg were cancelled, and the type was finally committed to action against German armour on 4 September. At first they had some impact on the German armour around Radomsko-Piotrkow and Pultusk, but as the number of aircraft dwindled the impact of their attacks also faded. On 9 September the active squadrons were ordered to collect 20 replacement aircraft from the conversion unit, No.213 Squadron, but only three of the nine aircraft were immediately ready for combat. The P.37 squadrons continued to fly operations until 16 September, before on 17 September, after the Soviet invasion from the east, the surviving aircraft were ordered to Romania.
During this brief campaign the 45 P.37s flew just over 100 sorties between them, dropping 330,700lb of bombs. They were also credited with the destruction of six Bf 109s. However 26 were lost during the battle. The P.37 was about on a par with the versions of the Heinkel He 111 used in Poland, and far superior to the Dornier Do 17Z, carrying twice the payload at higher speeds. However the Germans had hundreds of both types, compared to the handful of P.37s.
The remaining 19 aircraft from the Bomber Brigade and at least 20 from No.213 Squadron, reached Romania, where they were slightly modified then pressed into service. They were used on the Eastern Front from 1941-44, and remarkably some of the survived, with at least one still in use as a target tug in the mid 1950s.
Work began on a more advanced version, the P.49 Mis, but by September 1939 only the wings and parts of the fuselage of the first prototype had been completed.
Engine: Two Pegasus XX radial air cooled engines
Crew: Pilot, radio operator, navigator/ bomb aimer, dorsal gunner
Span: 58ft 10.25in
Length: 42ft 4.75in
Height: 16ft 8.25in
Empty Weight: 9,436lb
Normal loaded weight: 18,872lb
Maximum loaded weight: 19,621lb
Maximum Speed: 276.5mph at 11,154ft; 242.5mph at sea level
Service Ceiling: 19,685ft (normal loaded weight), 30,347ft (without bombs)
Range: 932 miles with 4,850lb bombs; 1,615 miles with 3,880lb bombs; 2,796 miles with no bombs
Bomb load: Up to 4,850lb