1) A first client to take a chance on you, because no one wants to be the first client, very few companies will buy something that no one else has bought. In my experience your first client (or first few) is the result of some existing personal connection to the client (i.e. a favour/referral), and expect to be at their beck and call for a while, and probably take a bath on the price.
2) You need an experienced sales team that knows how to sell to enterprise/B2B. This is not something you can "pick up" or learn on the job (or at least is very difficult to do). You need to hire someone who has done it before, otherwise you will stumble and spend your way through months-long sales cycles only to have them fail at some late stage because you didn't know how to prepare.
3) You likely need standards certifications and third-party testing. Look into ISO 27001 and other similar standards if you're a software company. Get third-party security testing done up front. Have pre-written answers and "white papers" around privacy, data governance, scaling, deployment, etc..
I've been on both sides of selling into enterprise and medium/large B2B and it's a solid 50/50 mix of your actual product capabilities and your company's ability to navigate procurement and due diligence.
It also depends on what you're selling of course. My experience is in data management solutions (including PII data), and standards/requirements can be pretty demanding in that world.
There is a trick to speed things up a little with Enterprise sales - find out who is an approved supplier for the enterprise you want to sell to and get them to “sell" your product for you under some sort of licensing deal, but where you actually do all the selling. This cuts out a huge amount of pointless activity dealing with said enterprise’s lawyers and purchasing people.
Also in my experience, being able to produce existing docs around your standards and processes, versus producing them on-demand, helps inspire confidence in the prospect's due diligence team, because it shows them you've thought of this before they asked about it. :-)
So I sent 100-200 personalized emails throughout a 4 month period to every single person I could find in the space. Eventually I got one and made a ton of connections just with that alone.
Now I'm also doing a startup and am talking with some big corporations just with getting to the right people and climbing my way to the top to sell my product.
Tyler Bosemany did a great talk about how to sell, i'd reccomend you watch it.
Another tidbit: Distribution kills a company, not the product.
Learn to sell (and make your product great) and everything will work out.