The short answer is that they are probably both often said by English speakers without too much thought to mean the same thing.
You would say "I intended to..." when speaking in the past tense to indicate your purpose or reason for doing something that occurred in the past. Perhaps you fulfilled that intention by doing what you meant to do, or perhaps you did not manage to fulfil it but still have the same intention.
When you say "I was intending..." to do something there can be the implication that this is no longer your intention, especially if you emphasise the word 'was'. It may be that, in your recounting of past events, you indicate that you had a particular intention but then something occurred to make you change your intention.
As with so many matters of grammar, context is everything.