There are lots of really useful advices on all other comments but there is something I haven't seen mentioned yet. I had those problem when building a SaaS targeted at small to medium business (up to 200M annual revenue).
- Who is going to manage your software inside the business?
In my case we expected that by now almost all of those companies would have internal IT personnel or at least some qualified IT partners but it wasn't based on anything... and it wasn't true, this was a major pain point. We had to create a "sibling" company that handled user training, implementation, integration and many other things because most of our customers couldn't do this themselves. As you might understand customer spend on those services is higher than that on the actual SaaS but with way lower margin, think of Salesforce for example: many SF customers spend way more in partners than on software licences, and many if not most SF customers need a partner to implement SF and cannot do it themselves.
- Which problem are you trying to solve for those companies? Is this problem linked to other stuff?
When you start your project it may seem that it is a perfect fit and solves all the problems you are imagining, but as soon as you start selling you notice that those problems you are solving are just the tip of the iceberg. In a short time you may find yourself with a software that actually only solves 5% of the bigger problem and now there is a 95% of unknowns that must be addressed. Maybe in those unknowns there are other SWs, maybe legacy SWs, and you need to communicate/integrate with them or else your offer doesn't make sense to the customer.
From my anecdotal experience you need to find some initial customers that look like your potential target customer and sell them your idea, maybe having just a simple prototype, and only after you have a few you start building for real keeping those early adopters in the loop. If you're not a seller find a sales co-founder, sales are extremely important when dealing with business.
Please note: I don't know what you are building so those adivices might be totally irrelevant, I hope they can help you anyway :)