The most capable medium bomber of the day was the twin-engine Vickers Wellington, which first flew in 1936 and entered RAF service with No.99 Squadron at Mildenhall in October 1938. The production aircraft bore little resemblance to the prototype aircraft and compared to contemporary medium bombers already in service, the Wellington appeared to be much more advanced in design and an aircraft feared by any potential enemy. Its sleek monoplane design and heavy defensive armament placed the new Wellington as one the advanced and capable medium bombers in the world.
Perhaps the most significant feature of the Wellingtons design was the adoption of a geodetic construction method, which was developed by famous British engineer and inventor Barnes Wallis. Duralumin W-beams were used to form a metal lattice-work construction on to which wooden battens would be screwed, which would then allow the doped fabric outer skin of the aircraft to be attached. 
The resultant fuselage was relatively light in weight but possessed great strength and whilst the method o construction posed challenged for companies engaged in manufacturing Wellington bombers, the inherent strength proved crucial when the aircraft was thrust into combat. 
Capable of withstanding significant battle damage, numerous RAF Wellingtons managed to bring their crews back home, when the other bombers would have failed to do so.