As only a handful of aircraft had been introduced into the French Air Force by the outbreak of the Second World War, the LeO 45 was too late to provide a substantial contribution during the Battle of France in the face of an invasion by Nazi Germany.

It was quickly determined that existing aircraft would be incapable of performing the latter role to a sufficient degree, thus a modernisation programme, known as Plan I, was promptly initiated.

[2] In addition, several shortcomings was quickly identified with many of these aircraft, including inadequate defensive armaments, being too slow and relatively unmanoeuvrable, and thus too vulnerable for viable sorties over hostile territory.

[2] On 17 November 1934, the Service Technique Aéronautique released the B5 heavy bomber programme and made approaches to all of the main French aircraft manufacturers.

The new bombers would have to be capable of operating at day and night, while carrying defensive turrets, the forward unit being fixed to the fuselage while the ventral position was to be flexible.

[4] During the latter half of 1935, the Service Technique Aéronautique decided to enact several changes to some of the specified requirements; these included the revision of the rearward and lower turret arrangements and the crew reduced from five to four, as it was thought that the co-pilot could also serve as a navigator and bomb-aimer.

[4] In September 1936, the envisioned bomber's requirements were revised to account for development of 1,000 hp (746 kW)-class engines; accordingly, the cruise speed raised to 470 km/h (255 knots, 290 mph).

On 26 March 1938, a further 20 LeO 450 was ordered in line with the French Air Ministry's newly adopted Plan V, which called for the reequipping of 22 bomber units.

[11] During early 1939, as it was becoming apparent that the international situation was worsening and that the European powers were increasingly likely to be involved in a major war, the Armée de l'Air explicitly requested of SNCASE that the company would not delay production by enacting any further improvements; the mandate was issued in spite of known teething troubles with the type that were far from cleared.

[13] While additional productions lines were established in order to produce more aircraft, much of this extra capacity did not arrive until mere months prior to the catastrophic Battle of France, thus making their contribution to the course of the war minimal.

The LeO 45 featured an all-metal construction and a monocoque fuselage; the structure included 60 individual fixed frames attached to longitudinal stringers and was covered by flush-rivetted light alloy panels.

A corridor alongside the main bomb bay led to the dorsal gunner's position, which featured a powered mounting for the 20 mm cannon Hispano-Suiza HS.404, with 120 rounds; the turret could be retracted.

[10] The undercarriage was fully retractable; it featured an unusually complicated mechanism for the main wheels in order to reduce the size of the engine nacelles.

The two horizontal surfaces of the tail unit were constructed in two separate halves and bolted onto a short center section fixed to the upper fuselage; the twin fins and rudders, which were furnished with trim tabs, were attached at the extreme edges of the tailplane.

[18] At the start of the Battle of France on 10 May 1940, only 54 of the 222 LeO 451s that had been delivered were considered ready for combat, the remainder being used for training, spares, undergoing modifications and repairs or having been lost.

By the Armistice of 25 June 1940, LeO 451 of the Groupement de bombardement 6 (bomber wing) had flown 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 five lost in accidents.

LeOs were optimized for medium-altitude operations (5,000 m) but were forced to go far lower to search for and destroy tactical targets, rarely with even a basic fighter escort (H-75, D.520).

LeOs were not unarmed and German fighters had to keep a look out for their dorsal turret: on 6 June 1940, gunner sergeant Grandchamp, GB II/11 shot down two Bf 110Cs with the Hispano cannon.

Upon Italy's entry into the war on the Axis side, LeOs attacked Livorno, Novi Ligure, Vado and Palermo in a four-aircraft morning mission.

Turin (Fiat plants) was near the border and easy to reach, while Italy did not possess a radar system and radios were not commonly used on the Italian fighters.

The first of the newly produced aircraft flew on 30 April 1942, and by the time the German occupation of Southern France following Operation Torch brought production to an end, 102 LeOs had been built for Vichy.

[23] In 1942, LeO 451-359 was fitted with an experimental degaussing coil for remotely detonating naval mines[24] (some British Vickers Wellingtons and German Junkers Ju 52s also carried a similar device).

On 24 October 1940, Vichy French LeO 451s carried out an air raid against Gibraltar in retaliation for the Free-French attack on Dakar, losing one of their number to British anti-aircraft fire.

[25] 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.

The Germans were not especially interested in the type but on 21 May 1943, the Luftwaffe requested the Regia Aeronautica to hand over 39 LeO 451s captured by Italians troops in the SNCASE factory in Ambérieu-en-Bugey (Lyon).