PZL.46 Sum (sheatfish) was a light bomber of the Polish Air Force before World War II, which, was directed to serial production in the spring of 1939. These planes were in production, but the Polish industry did not manage to produce them before the outbreak of the war. Design and development The PZL.46 Sum was designed by Stanisław Prauss in the PZL works as a successor to the standard Polish light bomber and reconnaissance plane, the PZL.23 Karaś, also of Prauss' design. First sketches were made in 1936. In order to test new features, like double tail fins and a retractable underbelly bombardier gondola, a single modified PZL.23, designated PZL.42 was built and tested in 1936. The first prototype of PZL.46 Sum flew in August 1938. It shared only a general composition with Karaś, its fuselage was much more aerodynamically refined and its wings had thinner profile. Initially it was intended to use retractable landing gear, but since the Polish industry did not produce retractable landing gear, a fixed one was used. In November–December 1938 the prototype was shown at the Paris Air Show. In May 1939 the second similar prototype was flown (PZL.46/II). Tests proved, that the Sum was a successful design, with much better performance, than Karaś. The only major faults were with a mechanism of retracting of the underbelly gondola (the gondola for a bombardier, with a machine gun at the rear, was a feature copied from Karaś, where it was a fixed one. It was a rare feature in light bombers' construction and its usefulness was questionable, as it reduced performance). In March 1939 the PZL.46 was ordered for a serial production. The Polish Air Force planned to buy 160 aircraft of the PZL.46A variant. The first aircraft were to be completed in autumn 1939 and delivered in early 1940. Only some parts were produced by the German invasion of Poland in September 1939. The third prototype was in construction in the summer of 1939, it was to be an export variant PZL.46B, powered with Gnome-Rhône 14N21 engine. Bulgaria, using PZL.43 Karaś bombers, showed an interest in the new design and made a preliminary order for 12 aircraft, uncompleted due to war. Stanisław Prauss made also a preliminary design of smaller development variant PZL Łosoś, which was to be a two-seater dive bomber with Hispano-Suiza 12Z inline engine and retractable landing gear, without underbelly gondola. Technical