Final answer:
The Wright brothers, Orville and Wilbur, were the ones who closed a contract in 1908 with the US Department of War for the first military plane, following their notable achievements in aviation including the first powered flight in 1903.
Step-by-step explanation:
The individuals who closed a contract with the US Department of War for the first military plane in 1908 were the Wright brothers. After their successful flight in Kitty Hawk, North Carolina, in 1903, the Wright brothers made significant advancements in aviation. They honed their aircraft design, demonstrating flights, and incorporating their own company. Their work laid the foundation for both commercial and military aviation.
The Wright brothers' innovative control system and practical application of scientific theory in aerodynamics were critical in conquering the skies during the early 20th century. Their capabilities were recognized by the US Department of War, leading to the 1908 contract.
By contrast, other figures like Amelia Earhart, Charles Lindbergh, and Igor Sikorsky were also significant in aviation history but at different times and in different contexts.
Lindbergh is notably remembered for his solo transatlantic flight in 1927, and Earhart for her pioneering flights as a female aviator. Meanwhile, Igor Sikorsky is recognized for advancing helicopter technology. However, for the specific context of the first military plane contract with the US Department of War, the correct answer is the Wright brothers.