Is the loan principal ever delivered in pieces over time?
Handing out the principal amount of debt gradually, in increments, is standard practice in investment loans extended by a bank to a corporation.  

The rationale is clear : the corporation wants to make an investment, say a new factory. The whole plan is laid out and the cash flows of the pre-operational, construction period are also detailed, based on forecasts or specific agreements with the suppliers (contractors etc) that will be involved. The bank checks and accepts all these, but it also wants to see the corporation deliver (both the corporation's part in the financing but also as regards the actual build up of the investment).  

So specific milestones are agreed between the two parties, and to each milestone a round of financing is agreed (these milestones become contracutal obligations, legally binding). Milestones usually include specific technical phases of the build up to be accomplished (say, the building's foundations are complete, or the power infrastructure has been installed, etc). 

In this way risk is reduced without hurting the necessary cash flow for the project. It also reduces the interest burden for the firm.