I don't think there is a visa for something as short as a 6-month contract. The closest viable option I can think of is a "New Office" L-1 (intra-company transfer) visa. You'd have to open a new office in the US, and this is a longer term visa (5- or 7- years).
It's also relatively hard to get. You'll need the help of an immigration law firm to prepare the documentation needed to apply for the visa. The application process alone could take 6 months.
Also, just FYI: coming here as a visitor (on VWP/ESTA or the B-2 visa), then working (or doing a contract) here without permission would be violation of U.S. immigration laws, and can get you deported and effectively banned.
It's also relatively hard to get. You'll need the help of an immigration law firm to prepare the documentation needed to apply for the visa. The application process alone could take 6 months.
Also, just FYI: coming here as a visitor (on VWP/ESTA or the B-2 visa), then working (or doing a contract) here without permission would be violation of U.S. immigration laws, and can get you deported and effectively banned.