Answer the following Q with stream of consciousness. Context: The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the actual bombs. The Army component of the project was designated the Manhattan District; Manhattan gradually superseded the official codename, Development of Substitute Materials, for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US $2 billion (about $22 billion in 2016 dollars). Over 90 % of the cost was for building factories and to produce fissile material, with less than 10 % for development and production of the weapons. Research and production took place at more than 30 sites across the United States, the United Kingdom, and Canada.

Question: the u.s. manhattan project of 1945 was responsible for developing?
To answer the question, consider the following: The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. Therefore, the final answer is the first nuclear weapons.