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The first V-8 engine of record was not A De Dion-Bouton but was the Antoinette V-8 of 1903
Antoinette had a 180 degrees crankshaft did all others until Wright, who was making the Hispano-Suiza of 718 cu. In. and 1124 cu. in. in about 1918 or a later wanted to put the 90 degree crankshaft into the 1124” engine but the Navy and the Army Air Corp wanted air cooled Radials so this left it up to Cadillac to do it in 1923. This is recorded in Manuel Lage’s book about Hispano-Suiza page 66, Line 10. 
http://www.tecsoc.org/pubs/history/2002/dec2.htm
That design came from a Frenchman with a background in art, Léon-Marie-Joseph-Clement Levavasseur (1863-1922). He started work on his new engine in about 1900. His engine had eight cylinders, but instead of putting them in a straight line, Levavasseur arranged them in the shape of a ""V."" It was a powerful and compact design, and when he received a French patent on December 2, 1902, Levavasseur became the first person to patent what we now call a V-8 engine.

http://www.hubertlathamwindkiller.com/Monoplane.htm

The Antoinette engine was designed and built in France by Léon Levavasseur. Named after the designer's daughter, it was Europe's most widely used engine until 1909-1910. The first Antoinette engine dated from about 1901 and was used in a speedboat. By 1905, Levavasseur had produced a water-cooled engine with eight-cylinders arranged in a 90-degree ""V"" and with direct fuel injection. It was safe, strong, and fairly powerful, generating 50 horsepower (37 kilowatts) and weighing about 110 pounds (50 kilograms). Its power-to-weight ratio was not surpassed for 25 years

Antoinette VII. Designed by Léon Levavasseur, who was an artist as well as a gifted engineer, the Antoinette range possessed an almost art nouveau sense of elegance. The engine, also named 'Antoinette', came first, being designed by Levavasseur in 1903 to power motor boats. Both Santos-Dumont and the Voisins used this engine to power their early machines. The motor was of advanced design, utilizing direct fuel injection and evaporative steam cooling - which took place in long tubes fixed along the sides of the fuselage. 
   M. L. Anderson"