Prior to World War 2 (1939-1945), the nation of Poland held a prolific aero-industry responsible for several notable monoplane types emerging during the interwar period. The line of PZL fighters, and their unique gull wing mainplanes, began with the P.1 and was continually evolved until the fall of Poland at the hands of the German-led invasion to begin World War 2. Beyond fighters, the company also delved into the world of light bombers as showcased by the promising - though ultimately derailed - PZL.46 "Sum" ("Swordfish").
Development of the new light bomber began in 1936 and the design was intended to succeed the fleet of aging PZL.23 "Karas" platforms in same role for the Polish Air Force. As such, the PZL.46 shared some qualities with the earlier design and a test article was born from the existing PZL.23 stock as the "PZL.42". This led to a first-flight recorded in August of 1938 and four total prototypes were ordered to handle the various facets of development. One of the lot was to be set aside as a static testbed - the others to be flyable forms. Two of the flying trio (PZL.46/I and PZL.46/II) would house the British Bristol "Pegasus" radial engine while the third (export-minded model) would carry the French Gnome-Rhone 14N21 radial.
By March of 1939, the Polish Air Force planned on a fleet of 300 and an initial order of 160 was had as a second world war in Europe grew into a very real threat. In Polish service, these were to carry the formal designation of "PZL.46A" with deliveries beginning in 1940. Bulgaria, already operating the PZL.23 offshoot "PZL.43", became the first foreign customer of the design and secured an order for twelve which would be powered by the French engine and designated as "PZL.46B".
The aircraft followed conventional design wisdom for the time and incorporated modern elements such as an enclosed cockpit for its crew of three (the aft-gunner's position was partially exposed so as to clear the machine gun). The engine was positioned at the nose in the usual way with the cockpit sat over midships. The mainplanes were near-mid-mounted along the sides of the fuselage, given rounded tips and slight dihedral. The fuselage, well-rounded and streamlined, tapered at the empennage which was capped by a twin vertical fin tail unit. Construction was of all-metal with a semi-monocoque oval fuselage.
A fixed "tail-dragger" undercarriage, with spatted main legs, was used for ground running (a small, wheeled leg was fitted under the tail). As Polish industry lacked the know-how to manufacture retractable undercarriages, the fixed, spatted style was all that could be had for the design in the interim.
As built, the prototype had a running length of 34.4 feet, a wingspan of 47.10 feet, and a height of 10.9 feet. Empty weight reached 4,400lb with a Maximum Take-Off Weight (MTOW) of 7,825lb. Power was had from a PZL-Bristol Pegasus XXB 9-cylinder, air-cooled radial piston engine offering 918 horsepower driving a three-bladed propeller unit at the nose.
This installation, and the general design of the light bomber, provided the aircraft with a maximum speed of 265 miles-per-hour, a range out to 810 miles, and a service ceiling up to 25,300 feet. Rate-of-climb was 1,200 feet-per-minute. On the whole, the PZL.46's prototype proved the design sound and offered considerable advantages over the aging PZL.23 fleet.
Armament centered on a total of six machine guns: 4 x 7.92mm FK wz. 36 machine guns were in fixed, forward-firing mountings with two installed in the upper fuselage and synchronized to fire through the spinning propeller blades. The other pair was set in the wings. 1 (or 2) x 7.92mm Karabin Maszynowy Obserwatora (KMO) wz. 37 machine guns were carried on a trainable mounting at the rear cockpit. Another KMO, in a single installation operated by the bombardier, was featured at a ventral defensive station, interestingly this position being retractable (so as to preserve aerodynamic efficiency) and adding complexity to the bomber's design.
Beyond this, the aircraft was cleared to carry up to 1,325lb of conventional drop bombs under the wings.
Despite the rush to bring the PZL.46 into service, the bomber was not ready before the German invasion of September 1st, 1939. None were exported to Bulgaria from their twelve-strong order and only two flyable prototypes were ever completed for the host country. One prototype suffered undercarriage damage and remained in Warsaw while the other was relocated to Lwow on September 5th. On September 17th, this airframe escaped capture to Romania where it was interned.
On September 23rd or 26th, the aircraft was flown (under the guise that it was to be delivered to Romanian aeroplane maker IAR at Brasov) by PZL test pilot Stanislaw Riess and three other crew into the Polish capital of Warsaw, his cargo being critical Polish resistance/defense documents. From there the trio escaped capture by flying to Kaunas, Lithuania before abandoning the bomber for a ship to take them to English shores and safety. The example in Lithuania then fell to the Soviets who tested it until its usefulness was expended.
The PZL "Losos" was a related development of the PZL.46 project, drawn up by Stanislaw Prauss as a more compact version of the light bomber. Its crew was reduced to two operators and the wheeled undercarriage made wholly retractable. Beyond this, the ventral retractable gondola was deleted for simplicity's sake and power would come from a Hispano-Suiza 12Z inline piston engine offering 1,600 horsepower. The form only existed as a preliminary design and went no further.