In 1929 the German Reichswehr (State Defense) issued specifications for a night bomber, as military aircraft production in Germany was still prohibited, the Reichsverkehrsministerium (RVM, State Transport Ministry) officially ordered the type as the two-engined transporter Do F. The prototype of first German developed and built bomber after WWI, flew for the first time on May 7, 1932, registered as D-2270, s/n 230. It was powered by two 550 hp Siemens-built Bristol Jupiter VI nine-cylinder radials.
Initial testing took place at the RDL at Staaken, while military equipment installation and subsequent flight tests took place at the secret military flight center at the Russian AFB Lipetsk, near Voronezh, Russia, that was used by the Germans from 1924 to 1933 (initially for training, later also for testing). In the summer of 1933 the prototype was redesignated Do 11 A.
In an act to camouflage the true purpose of the type the Deutsche Reichsbahn (DRB, German State Railways) wanted to operate the type in conjunction with Lufthansa as an air freighter. Before that took place, a small batch of "transporters" were delivered to the clandestine Luftwaffe via the DRB, subsequently they were fitted a bomb-aiming position, bomb racks, and three 0.312 in (7.92 mm) MG 15 machine guns. Fitted with two 650 hp Siemens & Halske Sh 22 (later known as SAM 22 B) nine-cylinder radials, these aircraft were designated Do 11 C.
Due to unsatisfying flight characteristics the next type, the Do 11 D, was fitted with wings of reduced span and redesigned wing tips and was powered by the same Sh 22. As continuing problems with the retractable landing gear mechanism could not be solved the landing gear remained locked in the extended position. The type was shown in public for the first time on May 1, 1934, and at least 77 were produced of which 10 were operated by the DRB: D-ABEL, D-ABEX, D-ABOS, D-ADAN, D-ADUL, D-AFEZ, D-AGIF, D-AHER, D-AJOL and D-AZUN.
The flight characteristics of the Do 11 remained unsatisfying, so the type was significantly redesigned and fitted with wing of larger span, new rudder, flaps and a fixed landing gear, while two 530 hp Jupiter VIs replaced the 650 hp Sh 22. Designated Do 13 A this prototype flew for the first time on February 13, 1933. Two more prototypes were produced before the Do 13 C appeared, fitted with two 750 hp BMW VI twelve-cylinder liquid-cooled V-engines. A number of Do 13 Cs were lost due to structural failures in the wings. A Do 13 D and a Do 13 E were tested unsuccessfully as well.
After the Do 11 and Do 13 proved unsatisfying and a replacement was not available yet, a redesigned series of the bomber was produced with the designation Do 23 F. This type was distinguished from its predecessors by BMW VI U engines, wings with reduced span, a reinforced fuselage, and auxiliary stabilizing surfaces beneath the tail plane. Flown for the first time on September 1, 1934, the changes proved to be successful and a batch of 36 aircraft (c/n 351 to 386) was produced.
A number of small additional changes were made leading to the designation Do 23 G from c/n 387 on, and in all 272 Do 23s were produced from 1934 to 1936 by Dornier at Manzell (Friedrichshafen) and Wismar, Henschel at Berlin-Johannisthal, and Hamburger Flugzeugbau at Hamburg. The Do 23 was supplanted by the Ju 86 and He 111 from 1938 on, being deregulated to air gunnery and bombing trainers.
Do 11 D
Cristian Savin Collection Do 23 F
Cristian Savin Collection