They say it's the largest ransomware attack ever made, NSA is guilty of this because hackers apparently used their tools to spread it. https://intel.malwaretech.com/botnet/wcrypt
No. The NSA is not the guilty party here. Microsoft is. Security is not the primary design goal of Windows. DRM and the protection of "intellectual property", followed by up-selling are the primary design goals of Windows.
Was Windows XP the only OS affected? Or the newest ones too?
Most of the affected systems had Windows XP (perhaps more than 90%). It was quite irresponsible from the part of Microsoft to end their support for Windows XP. The severity of this particular attack could have been significantly reduced if there was support.