Oh, I see! Yet, "Będę pisał" could also mean "I will" as well as "I intend". Future in Polish though is admittedly more definite than in English. When Polish declares that something WILL be done, it is as if it already HAS BEEN done, I think-:)
I am going to be splitting hair a bit here but for me there is definitely big difference between the two: napiszę i będę pisać.
Napiszę - napiszę list, napiszę książkę - is a promise that I am going to make it and the result will be a finished product - no matter how long it will take: one hour, 10 years, or all my life. And it does not need to be done during one session. So all the theories about timings are not that important here. The important part is "dokonanie" - the act of finishing of the action of writing. No one expects me to write a book in one hour, but "słowo się rzekło, kobyłka u płota" and my listeners assume that I really intend to finish the thing up, and I will finish it - unless I am a perpetual liar.
Using the perfective in past tense is even easier, because your audience may demand a verification from you. "Napisałeś wreszcie tę książkę? Pokaż!" Show us a copy or prove that it is available in bookstores or libraries. Yeah, yeah - we know that "ty ją pisałeś, ale tak naprawdę nigdy jej nie napisałeś i nigdy nie zamierzałeś jej napisać."
In "będę pisać", however, there is no such promise of "dokonanie". "Będę pisać książkę o Ameryce" - does not contain any qualification about the final result. It is vague; I may never even intend to finish it up. This might be my way of dealing with boredom.
"Będę pisał listy do ciebie" - contains a promise of writing, but it is an open promise, with no definite end.
"Przyrzeknij mi, że będziesz do mnie pisał jeden list dziennie." This sentence is vague, because it may well mean a process of writing, abandoning it in the middle - never mind posting it out. The correct form would be "Przyrzeknij mi, że napiszesz do mnie jeden list dziennie." However, such form is never used because a rational supplicant cannot actually expect "dokonanie" in every circumstances. So she begs to at least try writing to her (Sorry, Mom).
The past form "pisałem", again has no promise of "dokonanie". As someone already say here: "Pisałem list, gdy Jurek wszedł do pokoju." The letter writing was interrupted and there is no promise that it was ever finished.
"Pisałem tę książkę przez wiele lat, aż mi się to znudziło". Again, no final product, but the implication is that the process was repetitive, routine - maybe one page a day, or one page a month.
I do not remember how is it formally defined but there is one more form for all three tenses:
In all this cases the act of writing is sporadic, maybe random, from time to time.
"Pisywałem także do gazet sportowych – 'Barwy sportu', 'Gem, set, mecz', itp.
"Będę pisywał do ciebie od czasu do czasu."
"Gdzie ludzie listy pisują?"
"Jestem staroświecka jak Jan III Sobieski, pisuję słodkie listy miłosne."
Try to substitute "pisuję" by "piszę" in the last sentence and you should intuitively see that the effect is not quite the same. The lady here is not in a habit of writing love letters routinely; she does it from time to time.