Design development of what was to become the Do 23 began late in 1931, at which time the aircraft was known as the Dornier F, and the first prototype, powered by two 550 hp Siemens Jupiter nine-cylinder radials, flew on May 7, 1932, attaining a maximum speed of 155 mph (250 km/h). Although the military role of the Do F was quite obvious, the German State Railways decision to operate an air freight service in conjunction with Lufthansa provided an opportunity to present the aircraft publicly, although its commercial veneer was thin and fooled few. The first few series production aircraft, redesignated Do I 1, featured a loading hatch in the top of the fuselage immediately aft of the wing trailing edge, the fuselage itself having been modified to provide a small freight compartment. In this form, the Do It was revealed publicly on May 1. 1934, but what was not revealed was the fact that every aircraft delivered by Dornier to the State Railways was accompanied by an assortment of sealed crates, containing such items as an interchangeable fuselage nose containing a bomb-aiming position. bomb racks, and machine gun mountings.

As the Do 11C the aircraft equipped the first bomber formations of the still clandestine Luftwaffe. The aircraft was soon found to leave much to be desired. The wings tended to vibrate violently under certain flight conditions, necessitating an order forbidding turns demanding a sharper banking angle than 45 Grad and the poor reliability of the undercarriage retraction mechanism necessitated this being locked in the extended position. In an attempt to itnprove the characteristics of the aircraft, the wing was reduced in span by 2,7 m and the wingtips redesigned, and aircraft on the assembly line that received these modifications were redesignated Do 11D, although the same 650 hp Siemens Sh 22B-2 radials were fitted, and all serving Do 11Cs were brought up to the same standards.

Adverse reports on the characteristics and capabilities of the bomber led its manufacturers to admit that it was, perhaps "technically somewhat ahead of its time", and design of a simplified version was initiated, the Do 13 with a fixed undercarriage. Designated Do 13a, the first prototype was flown on February 13, 1933, and employed an essentially similar airframe to that of its predecessor, and two further prototypes, the Do 13b and Do 13c, were built before series production was initiated as the Do- 13C with two 750 hp BMW VI liquid-cooled in-line engines. Soon after deliveries began several Do 13C were lost as a result of structural failures in the wings, and further redesign was undertaken, although the new model, designated Do 23, differed little from its predecessor, apart from a decrease in wingspan from 28 m to 25,6 m and the provision of small auxiliary stabilising surfaces beneath the tailplane. It was, however, structurally strengthened, and in 1934 the aircraft entered production as the Do 23F, this initial model being followed by the improved Do 23G which, built in relatively large numbers, equipped many Luftwaffebomber formations until it finally gave place to the Ju 86 and He 111 in 1937- 38. Do 23Gs subsequently served in the roles of air gunnery and bombing trainers, while a number of the older Do 11Ds were delivered to the Bulgarian Air Force and. in fact, used operationally by that air arm alongside Luftwaffe aircraft in 1941. The Do 23G was also fitted with a degausing ring for anti-mine patrols in the Baltic.

With two 750 hp BMW VI U liquid-cooled engines, the Do 23G attained a maximum speed of 260 km/h, normally cruised at 210 km/h , had an initial climb rate of 4,5 in/sec, attained an altitude of 1 000 m in 4 mm, possessed a service ceiling of 4200 m, and had a maximum range of 1 350 km. Empty and loaded weights were 5 600 kg and 8 750-9 200 kg, and overall dimensions were: span, 25,6 m, length, 18.8 m height 5,4 m, wing area, 106.6 m2 Maximum bomb load was 1 000 kg, and defensive armament comprised three 7,9 mm MG 15 machine-guns in open nose, dorsal and ventral positions.

Dornier Do F, 11, 13, 23
This article is from Flying Review International  Jan. 1966
Do 11
Do 23G
Do 23G