ISO 9241-210 says user experience is “a person’s perceptions and responses that result from the use or anticipated use of a product, system or service”. It includes all the users’ beliefs, perceptions, preferences, emotions, physical/psychological responses that happen before, during and after use. (See also UX on Wikipedia)
Given that B2B decisions are also taken by humans, your website UX needs to be whatever it takes to make those humans feel good about starting a business relationship with you. That includes careful, and usable website design and presentation, attractive and convincing product descriptions and presentations, usable sales and backend processes and experiences, and so on. In a nutshell, the feelings you want to evoke are different between B2C and B2B, but UX has the same importance in both of them.