A suspended slab will do the trick for you, and there are many ways to accomplish this type of roof.
You can use prestressed precast concrete panels although 40 feet may be at their outer limits. I haven't used them in awhile and would have to check. We did do a 36 foot clear span garage with them though. Depending on where you are located and the loads on the roof this can be a fast no fuss no muss way to go.
Cast in place is a viable option also and here you would have many options to choose from. ICF system, conventional wood forming, a commercially available forming system used in high rise construction, and or post tensioned concrete to name a few. For some one who has never done suspended slabs the ICF systems offer a simple and proven route to accomplish your goal. The commercial systems are set up for speed and efficiency but you usually have to have big equipment to move them around. Post tensioning is great but you have to have the contractors available and the access to the building site for the post tensioning process.
Having the right engineer is key to the success of your endevour. You need to find an engineer that has a good amount of experience with suspended slabs. Don't go to the first name you come to in the phonebook a say great I found my engineering firm, AAA Engineering We engineer all kinds of stuff call 555-1212. This would be certain path to disaster.
Find someone who will listen to your wants and desires for your roof then let them come up with your needs and requirements of a system. This will ensure a smooth building process.