The LeO 45.01 B4 – designed by Jean Mercier to Armee de l’Air Programme A21 (1934) – first flew on 16 January 1937. It was an all-metal low-wing monoplane powered by two radial engines. The wings had considerable dihedral and the streamlined elliptical monocoque fuselage had a pointed and fully glazed nose. The landing gear was fully retractable. Accommodation provided for a pilot in an enclosed cockpit, behind which was the radio operator’s panel and below the retractable ventral gun turret.

Production LeO 451s had two 849.5kW Gnome-Rhone 14N 48/49 or 38/39 radial engines in specially designed Mercier cowlings. Despite excellent performance, construction of the type was slow. Only five LeO 451s were on first-line strength by 3 September 1939. When France collapsed in June 1940 only 452 of some 1,700 ordered had been delivered. A number of modifications were incorporated during series production, the principal being a totally redesigned fin and rudder assembly. Armament included a fixed 7.5mm MAC 1934 nose machine-gun, another gun of the same type flexibly mounted in the ventral gondola, and a somewhat troublesome 20mm HS-404 on a special mounting in the dorsal position. Maximum bomb load – carried in fuselage and wing bomb bays – was 2,400kg.

The first production LeO 451 was built in 1938. The decision to abandon Hispano-Suiza engines and a shortage of propellers resulted in production delays. The latter also caused most aircraft to be fitted with slower Ratier propellers which reduced the top speed from 500 to 480 km/h. As the result, although 749 LeO 451 had been ordered, only 22 were delivered by the start of World War II. Of these, only 10 were formally accepted by the Air Force.

At the start of the Battle of France on 10 May 1940, only 54 of the 222 LeO 451 were considered ready for combat, the remainder being used for training, spares, or undergoing modifications and repairs. The first combat sortie was flown by 10 aircraft from GB I/12 and GB II/12 on 11 May 1940. Flying at low altitude, the bombers suffered from heavy ground fire with one aircraft shot down and 8 heavily damaged. Within the next 8 days many of them were shot down, like the one driven by sergent-chef Hervé Bougault near Floyon during a bombing mission over german troops. By the 25 June 1940 Armistice, LeO 451 of the Groupement 6 flew approximately 400 combat missions, dropping 320 tons of bombs at the expense of 31 aircraft shot down by enemy fire, 40 written off due to damage, and 5 lost in accidents. A total of 452 aircraft had then been built, 373 accepted into service (including 13 for the Aéronautique navale), and around 130 lost in action in Europe.

Following the Armistice, LeO 451 continued to fly, now under the Vichy government. The aircraft were fitted with larger rudders and two additional 7.5 mm machine guns in the rear turret, and an additional 109 aircraft were manufactured under the Vichy regime. The most notable of these was LeO 451-359 which was fitted with a large degaussing coil for remotely detonating naval mines (some British Vickers Wellingtons and German Junkers Ju 52s also carried a similar device).

After Operation Torch which began on 8 November 1942, surviving French LeO 451 in North Africa were used primarily for freight duties, although they flew a few bombing missions against Axis forces during the Tunisia Campaign. They were ultimately replaced in active service by Handley-Page Halifax and B-26 Marauder bombers. Aircraft captured by Germans in occupied France were also primarily used as transports, although a small number saw action with the Italian Regia Aeronautica.

Following the war, the 67 surviving aircraft were used primarily as trainers and transports. LeO 451 was finally retired in September 1957, making it the last pre-war French design to leave active duty.

Variants

LeO 45
First prototype, Hispano-Suiza 14Aa engines
LeO 451
Production version with Gnome-Rhone 14N engines
LeO 451E
Post-war flying laboratory, 11 modified
LeO 451T
German-captured bombers modified for freight duty, seating for up to 17 troops.
LeO 453
Post-war conversion to high-speed transports and search-and-rescue aircraft, Pratt & Whitney R-1830-67 engines, seating for 6 passengers, range 3500 km (1,890 nm, 2,175 mi) at 400 km/h (215 knots, 250 mph) cruising speed, 40 modified.
LeO 454
Bristol Hercules II engines, one prototype left unfinished.
LeO 455
High-altitude version with turbosupercharged Gnome-Rhone 14R engines producing 1,375 hp (1025 kW) each, 400 ordered, one prototype built. The aircraft flew on 1939-03-12 but was later destroyed on the ground.
LeO 455Ph
Post-war photoreconnaissance variant, SNECMA 14R engines with 1,600 hp (1195 kW) each, 5 modified.
LeO 456 (LeO 451M)
For French Navy, 68 ordered
LeO 458
Wright GR-2600-A5B engines, 10 ordered

Comparable aircraft

Bristol Blenheim
Dornier Do 217
Douglas DB-7
Heinkel He 111
Junkers Ju 88