Dornier Do. F / Do-11 / Do-13

During the late 1920s the German Dornier Metallbauten set up a subsidiary at Altenrhein in Switzerland to build heavy aircraft expressly forbidden under the terms of the Versailles Treaty. The Do P had four engines, the Do Y three, and the Do F was a large twin. All were described as freighters, but their suitability as bombers was obvious. In late 1932 it was boldly decided to put the F into production at the German factory at Friedrichshafen, the designation being changed to Do 11. The Do F, built at Dornier's Altenrhein factory in Switzerland, and which first flew in May 1932, powered by two 550-hp Siemens-built Bristol Jupiter radials and having a retractable main landing gear.
The Do 11 had a slim light-alloy fuselage, high-mounted metal wing with fabric covering carrying two 484.4kW Siemens Sh 22B engines (derived from the Bristol Jupiter), and a quaint retractable landing gear whose vertical main legs were laboriously cranked inwards along the inner wing until the large wheels lay flat inside the nacelles. There was obvious provision for a bomb bay and three gun positions.

In 1933, well before the official establishment of the Luftwaffe, an official order was placed for a production version of the Do F with two 650-hp Siemens Sh 22B-2 nine-cylinder radials, driving three-blade wooden propellers. They were delivered from late 1933 as Do 11Cs to the German State Railways which under the cover of a freight service actually enabled the embryo Luftwaffe to begin training future bomber crews. Although they were so used for freight, gun installa-tions, bomb racks and other military items were delivered separately and secretly. Thus they could be converted quickly into bombers and were openly in service as such in early 1934 with the Befeffiskampfgeschwader (auxiliary bomber wing) of the Luftwaffe.

The Do 11C was a twin-engined, high-wing monoplane, with an all-metal fuselage and a glazed observation section in the nose. The crew consisted of a pilot, wireless operator and two gunners, the latter sharing the three single 7.9-mm (0.311-in) MG 15 machine-guns, one each in dorsal and ventral positions and one on a ring mounting forward of the front cockpit.

The Do 11C was a contemporary of the Ju 52/3m in the initial formation of Luftwaffe bomber squadrons.

It had been planned to deliver 372 Do 11 in 1934 but delays, plus grossly unpleasant handling and structural qualities, led to the development of the Do 11D with modified, shorter-span wings, and the Do 11Cs in service were also converted to this standard. This was further developed into the Do 13, by fitting non-retractable landing gear and doing away with the glazed nose and the small auxiliary horizontal stabilizers fitted under the Do 11's tailplane.

At least 77 Do 11D were delivered, some later being passed on to another clandestine air force, that of Bulgaria.

The Do 13 was first flown on February 13, 1933, with the same powerplant as its pre-decessor. Substitution of Junkers-type 'double-wing' trailing-edge flaps, similar to those of the Ju 52/3m, gave it improved landing characteristics, and when the Sh 22 engines were replaced by a pair of 750-hp 12-cylinder liquid-cooled BMW VIUs, driving four-blade propellers, the bomber was redesignated Do 13C.

The Do 13 with fixed (often spatted) landing gear was wholly unacceptable, but in September 1934 testing began of a completely redesigned machine called Do 13e with stronger airframe, Junkers double-wing flaps and ailerons and many other changes. To erase the reputation of its forbear this was redesignated Do 23 and in March 1935 production restarted of Do 23F bombers.

No attempt was made to disguise the function of the bomber: the fuselage having a glazed nose for visual aiming of the 1,000kg bomb load housed in vertical cells in the fuselage, and nose, mid-upper and rear ventral positions each being provided with a 7.92mm MG 15 machine-gun. After building a small number the Dornier plant switched to the Do 23G with the BMW VIU engine cooled by ethylene-glycol. By late 1935 more than 200 had been delivered and these equipped the first five named Fliegergruppen - although about two-thirds of their strength comprised the distinctly preferable Ju 52/3m. Although it played a major part in the formation of the Luftwaffe and continued to the end of World War II to serve in training, trials and research roles, the Dornier Do 23 was not much better than its disappointing predecessors.

Do 11D
Span: 26.30 m (86 ft 3.5 in)
Length 18.80 m (61 ft 8.25 in)
Gross weight 8200 kg (18078 lb)
Maximum speed: 260 km/h (161 mph).

Do-13
Engine: 2 x BMW VIU, 540kW
Take-off weight: 8750-9200 kg / 19291 - 20283 lb
Empty weight: 3150 kg / 6945 lb
Wingspan: 25.6 m / 83 ft 12 in
Length: 18.8 m / 61 ft 8 in
Height: 5.4 m / 17 ft 9 in
Wing area: 108.0 sq.m / 1162.50 sq ft
Max. Speed: 260 km/h / 162 mph
Cruise speed: 210 km/h / 130 mph
Ceiling: 4000 m / 13100 ft
Range w/max.fuel: 1200 km / 746 miles
Armament: 3 machine-guns, 1000kg of bombs
Crew: 4