Question: How could a really intelligent species be kept from developing? For clarification, this species is the only creature of intelligent thought on the planet and they have a population of about 50 million across the planet about the size of the moon. However, they can't develop to the level that Britain reached in the industrial revolution.

Answer: 
### Imagine this:

The year is 2222 and human technology has advanced to the point that we are able to reach and colonize other planets. Life on Earth is thriving and things are going really well. However, there is still one problem. **The Amish.** They are still living their lives as they always have, but now they're using space that the government wants to use for other things. Invoking eminent domain, the government buys all of their land and, in return, transports all of the Amish to a (relatively) nearby planet on which they can all live. After receiving the means by which to build up life as they had it on Earth, the colony is never touched again, allowing the residents to remain there leading their simple lives.

**While this situation would almost never happen, there are some points it brings to light.** Just because a society has not reached a certain level of technology does not mean that they are unable to do so. As the Amish choose to live a life without many of the luxuries of modern life, another society could choose not to pursue further technological advancements for many reasons besides necessity. Maybe they have a religion that forbids the use of a certain material necessary for advancing technology. A scientist in their distant past may have caused a great war with his inventing, so a ban has gone out preventing certain technology. The society may just be content with life as they have it. While this information may not all prove to be useful to you, considering the fact that societies can be primitive by choice is a worthwhile one in my opinion.

*Note: I have absolutely nothing against the Amish, and instead merely consider the more general suppositions to still be interesting ones to consider.*
Have an existing spacefaring species from a different planet/solar system/whatever give opposing political factions nukes, tell them "the other guy" is going to kill them by the end of the day if they don't shoot first, and don't tell any of them how powerful the nukes are.

This will likely kill everyone on the moon and possibly destroy the moon itself. Even if there are survivors, they will be too few to recover any time in the next millennia and likely too radiation diseased to survive the next decade. And the flora and fauna will likely not able to sustain the survivors, so they probably won't even survive the year. And that's if they can find potable water. Maybe they survive a month, depending on how much food and clean water they have stocked in stores and homes.

So, no more development.