MB 10: The law - The Magen Avraham wrote in the name of the Ridbaz that this is true only for commandments of Torah origin, but Rabbinic ordinances do not require intent. According to this, if one said any of the various blessings, which are all Rabbinic ordinances (except for the Grace after Meals), without intent to fulfill his obligation, he has nonetheless fulfilled it. However, from several places in the Shulchan Aruch, it appears that he disagrees with this, and so is implied from the explanations of the Gr"a to Siman 489, that no distinction should be made between Torah and Rabbinic commandments. Later on in Siman 489, note 8, the Magen Avraham wrote that the Shulchan Aruch rules that performance of the commandments requires intent; and if so, where a person did not have intent the first time, he must go back and do the commandment again. Even so he should not make a second blessing, because where the blessing is concerned we must worry about the minority opinion that says they do not require intent, and see in the Biur Halacha. Also be aware that the Chayei Adam wrote in chapter 65 that the requirement to go back and redo the commandment is in a situation where the first performance could have been done for some other reason - such as blowing Shofar, where we can say he was practicing, or the reading of Shema was for his own learning, etc. However, if one read the Shema as we do, following the order of the morning prayers, or one ate Matzah or blew Shofar or took a lulav, even though he did not have explicit intent to perform the Mitzvah, he has nonetheless fulfilled it - because that is why he is doing this action, for the sake of fulfilling his obligation. What he means to say is that where the circumstances prove that his performance is in order to fulfill his obligation, even though he did not have explicit intent this is sufficient. However, in another situation he would certainly agree that the obligation has not been fulfilled, as we see in the Tosfos to Tractate Sukkah 39a. All of this discussion concerns whether one fulfilled the requirements, having already performed the action. Clearly, in the ideal one must certainly concentrate before performing any commandment, and have specific intent to fulfill one's obligation with this action. And so agreed all the latter commentators in their works - see the Chayei Adam in chapter 21, and in the Derech HaChaim in the laws of reading the Shema, and in the Maasei Rav.
60:5. One who read the Shema, and did not (11) concentrate upon the first verse, which is "Shema Yisroel [Hear, O Israel]", has not fulfilled his obligation. If one did not concentrate for all the rest of the Shema, even if he was merely studying a Torah Scroll or (12) correcting these sections at the time for the Shema, he has fulfilled his obligation - provided he concentrated upon the first verse.
MB 11: Concentrate - The "intent" referred to here is not that mentioned above in 60:4, which was intent to fulfill one's obligation - that is required throughout all the paragraphs of the Shema. Here, the "intent" is sincere concentration, and placing upon one's heart exactly what one is saying - and this is mandatory only for the first verse, which has within it the critical items of acceptance of the yoke of Heaven and the unity of HaShem. There are those who say that even the intent to fulfill one's obligation is only mandatory in the first verse.