One possibility using the titlesec package:
\documentclass{book}
\usepackage{titlesec}
\usepackage{lipsum}% just to generate text for the example
\titleformat{\chapter}[display]
{\normalfont\bfseries}{}{0pt}{\Huge}
\titlespacing*{\chapter}
{0pt}{20pt}{40pt}
\begin{document}
\chapter{The flight of the Bumblebee}
\lipsum[4]
\end{document}

Without packages, one can redefine \@makechapterhead as implemented in book.cls:
\documentclass{book}
\usepackage{lipsum}% just to generate text for the example
\makeatletter
\def\@makechapterhead#1{%
\vspace*{20\p@}%
{\parindent \z@ \raggedright \normalfont
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
\makeatother
\begin{document}
\chapter{The flight of the Bumblebee}
\lipsum[4]
\end{document}
In the previous example I decreased some vertical space before the title; if one doesn't want this, one can simply (as Mico suggests in his comment) \let \@makechapterhead (controlling formatting for headings for numbered chapters) to be \@makeschapterhead (controlling formatting for headings for numbered chapters):
\documentclass{book}
\usepackage{lipsum}% just to generate text for the example
\makeatletter
\let\@makechapterhead\@makeschapterhead
\makeatother
\begin{document}
\chapter{The flight of the Bumblebee}
\lipsum[4]
\end{document}
titlesecpackage for the changes. For example see: http://tex.stackexchange.com/questions/11444/how-to-format-the-chapter-heading . If you are still having problem, modify the question to be more specific. – mythealias Nov 05 '12 at 23:53