Of course I wouldn't deny that there was development in the early Church's understanding of Jesus, but as a simple matter of historical fact, the belief that Jesus was God originates very early in the Church, prior to any of the New Testament's composition. This is acknowledged by the vast majority of scholars.
At any rate, the fact that John's gospel has the most and clearest statements of Jesus' divinity simply does not mean that the synoptic gospels do not contain any such statements. For example, Jesus does say "I am" at some incredibly poignant places in the synoptic gospels. Below is Mark's account of Jesus walking on the water, with the Greek phrase "εγω ειμι" replacing the English. Bear in mind that Mark was probably the first gospel written.
When evening came, the boat was in the middle of the lake, and he was alone on land. He saw the disciples straining at the oars, because the wind was against them. About the fourth watch of the night he went out to them, walking on the lake. He was about to pass by them, but when they saw him walking on the lake, they thought he was a ghost. They cried out, because they all saw him and were terrified.
Immediately he spoke to them and said, "Take courage! εγω ειμι. Don't be afraid." Then he climbed into the boat with them, and the wind died down. They were completely amazed, for they had not understood about the loaves; their hearts were hardened.
The NIV translates the Greek as "It is I" here, as well as in the parallel description in Matthew 14, which concludes with his disciples worshiping him.
This is just one example; there are other interesting cases. Here are all the occurrences of εγω ειμι in the New Testament for your perusal.
(cross-posted at Quodlibeta)