Should first introductory paragraph explicitly state the subject of the paper? I am trying to write an introduction for a scientific paper. I am not sure, when I am announcing my subject, should I write that this is the subject, or just mention it without announcing it outright?

This is very confusing so here's an example.

a. Man was always interested by aliens. This paper discusses the possibility of alien life.

b.Man was always interested by aliens. Currently, scientific discoveries are shedding light on whether it exists.

Which phrasing is better? Is the first one too blunt?
The purpose of the introduction is to focus in on the specific, and likely more narrow, topic of your research from a larger perspective. the "larger perspective" is the larger scientific problem to which your study is tied. Therefore you can start the introduction by briefly explaining the larger perspective followed by identifying the existing gaps in knowledge and gradually work towards your own question. 

Many books on scientific writing compare the introduction to a funnel where the wider question is focused to the appropriate width of your research question. the main point is to set your study in a wider perspective so that you can tie your results into the gaps of the larger perspective. This helps readers to get a good perspective of your research and evaluate the results vis-à-vis existing knowledge.

Hence, your scenarios a and b look like a way to describe the writing strategy where in reality the two or three sentences usually requires one or a few paragraphs of text.