The Lioré et Olivier LeO 451 light bomber was one of the first modern aircraft ordered at the beginning of the 1930's and emerged as a very effective design which unfortunately suffered from engine problems which delayed its eventual production and introduction into service. Thus, only 10 Leo 451s were on hand when France entered the war and only three squadrons were equipped when the Blitzkrieg was launched in May 1940, far too little to make a difference. The LeO 451 would have performed admirably with good fighter cover, but with the Luftwaffe owning the skies over France the aircraft suffered heavy losses in daylight raids despite a surge in production. LeO 451s were continued to be built after the surrender, serving with Vichy Forces in the Mediterranean and Middle East, with the Luftwaffe, and even a small number were used as transports with the RAF and USAAF. Post-war, they remained in French service well into the 1950s.

The prototype LeO 45-01 was first flown on 16 January 1937. The main production variant was the LeO 451 which had uprated Gnome-Rhône radial engines instead of the Hispano-Suiza 14A of the prototype. After the surrender, production continued by SNCASE/Sud-Est with various other transport variants conceived such as the LeO 451C and 451T as well as the post-war Leo 453 conversion with Pratt & Whitney Twin Wasp engines.