Avia B.135; The Czechoslovak “Almost Was” That Became a Bulgarian Bomber Killer

October 6, 2021

The end of the First World War saw the creation of several new countries as the Ottoman and Austro-Hungarian empires were broken up by the victorious allies. One of these was Czechoslovakia.

Having a rather precarious existence in central Europe, sandwiched between several major and much bigger countries, the Czechoslovaks knew that if they were to survive, they needed to be able to defend themselves. Fortunately for them, they inherited a major arms industry which would be able to both develop weapons as good as any in the world and supply them in numbers. In this defence establishment was the aircraft maker Avia.

Avia built several biplane fighter designs in the 1920s, but as war clouds began to gather in the early 1930s the Czechoslovaks thought it wise to start building up their air force to a thoroughly modern standard.

The first product of this was the B-534, a rather fetching little biplane that, when it entered service in 1935, was as fine a fighter as any.


But the Czechoslovaks could see that the technology was moving on quickly and so, even as the new B-534s were entering service, they issued a specification for a next generation aircraft. In response, Avia began development of the B.35.

This was a monoplane fighter that utilized, like the Fokker D.XXI of the same time, a mixture of the new and old. The B.35 had a steel tube construction with metal skin from the cockpit forward and fabric on the aft fuselage. But in a nod to conservative thinking, the elliptical wing was wooden, and the undercarriage was of a fixed, spatted design.

It had initially been intended to fit an engine generating 1000hp, but this wasn’t available. So, the B.35, of which two prototypes were built, had the same Hispano-Suiza V12 engine as the B.534, which produced 860 hp.

Testing showed that despite some of its obsolete features, the B.35 was a highly manoeuvrable aircraft. Armament was intended to be a 20mm cannon shooting through the propeller hub and two machine guns – basically the same as contemporary French fighters of the era.

Ten preproduction aircraft were ordered and the B.35 looked like it would likely be the next Czechoslovak fighter. But even as this was occurring, Avia recognized that more could be wrung from the design by remedying the remaining issues.

So, they began work on the B.35/3 prototype.


This would replace the wooden wing with a metal, straight edged one, plus fit an outward retracting landing gear that stowed in the underwing.

In this design, the Czechoslovaks would have a modern fighter that was generally on a par with those of the rest of the world.

The only problem was all these events occurred in late-1938 and early 1939.

Abandoned by Britain and France to its fate, Czechoslovakia was occupied by Nazi Germany in March 1939 and the Avia factory found itself with some new, and not very welcome, overseers.

The B.35/3 first flew in June 1939, basically because of mild interest from the new German authorities and the advanced state of development. The German’s weren’t going to adopt the new aircraft, but one country within their sphere of influence was very interested.

The Bulgarians were looking to update their air force and took the opportunity of the German occupation to acquire a bunch of the seized Czechoslovak aircraft, including B.534s, from the Germans at a bargain price. But what they wanted was an aircraft they could build themselves, and the B.35s looked promising.

Bulgarian test pilots flew these in late-1939, and then the B.35/3 in June 1940. Impressed by the handling, the Bulgarians decided to adopt the aircraft, with the tacit permission of the German authorities, which had now been given the production designation of B.135.

This aircraft had some tweaks made to the flap design over the prototype, plus was fitted with an armament of a single Mauser MGFF 20mm cannon firing through the propellor hub, the originally intended French gun no longer being available. It also had two 7.92mm machine guns in the wings.

The engine remained the Hispano, which drove a two-bladed metal propellor, as all development on the proposed 1000-hp had been shelved. This gave the B.135 a top speed of 332mph (535kph) and a maximum ceiling of 8,500m (27,900ft).

The Royal Bulgarian Air Force placed orders for twelve aircraft, a production licence and for 62 of the Czech-built Hispano engines. These were intended to be used to produce another fifty aircraft in Bulgaria, which were to be designated as the DAR 11.

As it was, the production of these aircraft hit a snag when it proved beyond the Bulgarian factory to build them, and then the Germans stopped delivery of the engine after 35 because they needed Avia to build aircraft and engines for the Luftwaffe.

The twelve B.135s remained the only ones constructed, and their service record is rather patchy. Delivered by mid-1942, they appear to have been used as advanced fighter trainers and not seen any combat except for one recorded incident.

On the morning of March 30, 1944, four Avia B.135s were on a routine training flight just southeast of Sofia, the capital of Bulgaria. The group was flying at about 8000m (just over 26,000ft) when its lead pilot spotted the silhouettes of American B-24 bombers.

These were returning to Italy after conducting a bombing attack on the Romanian oil refineries at Ploiesti.

The Bulgarians promptly climbed up towards the B-24s, which must have been pushing the B.135s limits to be fair, and attacked them from beneath, passing through the formation. They then repeated the attack as they dove backdown through the Americans.

A third attack wasn’t possible. The Bulgarians reported that the B-24s climbed higher and accelerated, which put them out of reach of the fighters. Given the limitations of the B.135, by that point an old design and one for which new spare parts hadn’t been available for a couple of years, that’s completely understandable.

Plus, I can’t imagine the Bulgarians would have been very keen at throwing those aging and rather lightweight kites against a mob of riled up B-24s anyway, and I certainly wouldn’t blame them.

But as they tailed behind the American, apparently unsuccessful, one of the B-24s suddenly pitched over and spiralled to the ground far below, the crew bailing out.

And that was the one and only known victory of the Avia B.135. With the end of the war in Europe the following year, the B.135s seem to have vanished, no doubt wrecked or scrapped in the final days of the conflict.

Hence ends the story of the Avia B.135; an intriguing aircraft that might have proven quite successful in its time, had fate not intervened.

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