I want to incorporate startup selling only apps in Delaware as a c-corporation. All of the funders are US non-residents. As we will not be selling any material goods in US, will our corporation have to pay any income taxes in US (federal, state)?
Your accountant will probably tell you that the corporation will have to file a federal tax return. As a c-corp, taxes may have to be paid on any income. Most likely, the corporation will also have state taxes.
The fact your are not selling material goods may bear on sales tax, but probably does not affect your income tax liability.